Method for manufacturing semiconductor device

ABSTRACT

To improve productivity of a transistor that includes an oxide semiconductor and has good electrical characteristics. In a top-gate transistor including a gate insulating film and a gate electrode over an oxide semiconductor film, a metal film is formed over the oxide semiconductor film, oxygen is added to the metal film to form a metal oxide film, and the metal oxide film is used as a gate insulating film. After an oxide insulating film is formed over the oxide semiconductor film, a metal film may be formed over the oxide insulating film. Oxygen is added to the metal film to form a metal oxide film and added also to the oxide semiconductor film or the oxide insulating film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for manufacturing asemiconductor device.

In this specification, a semiconductor device means all types of devicesthat can function by utilizing semiconductor characteristics, and anelectro-optical device and a semiconductor circuit are semiconductordevices.

2. Description of the Related Art

A technique by which transistors are formed using semiconductor thinfilms formed over a substrate having an insulating surface has beenattracting attention. The transistor is applied to a wide range ofelectronic devices such as an integrated circuit (IC) or an imagedisplay device (display device). A silicon-based semiconductor materialis widely known as a material for a semiconductor thin film applicableto the transistor; in addition, an oxide semiconductor has beenattracting attention as another material.

It has been pointed out that by entry of impurities such as hydrogen, anelectrically shallow donor level is formed and electrons to be carriersare generated in an oxide semiconductor. As a result, the thresholdvoltage of a transistor including an oxide semiconductor is shifted inthe negative direction and the transistor becomes normally-on, so thatleakage current in a state where voltage is not applied to the gate(that is, in the off state) is increased. Thus, the entry of hydrogeninto an oxide semiconductor film is suppressed by providing an aluminumoxide film having a property of blocking hydrogen over the entire regionof a substrate so as to cover a channel region in the oxidesemiconductor film, a source electrode, and a drain electrode, so thatgeneration of leakage current is suppressed (see Patent Document 1).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2010-016163

SUMMARY OF THE INVENTION

In general, an aluminum oxide film can be formed by a sputtering methodor an atomic layer deposition (ALD) method. However, in the case wherean aluminum oxide film is formed by a sputtering method, powder aluminumoxide is generated in a treatment chamber of a sputtering apparatus.Entry of the powder aluminum oxide into a deposited film causesreduction in yield.

In the case where an aluminum oxide film is formed by an atomic layerdeposition method, trimethylaluminum (TMA) and water vapor arealternately introduced into a treatment chamber; thus, the time forformation is lengthened, which becomes a cause of reduction inthroughput.

In a transistor including an oxide semiconductor, oxygen in the oxidesemiconductor film is removed from the side surface of the oxidesemiconductor film and oxygen vacancies are formed by etching treatmentfor etching the oxide semiconductor film into a desired shape, exposureof a side surface of the oxide semiconductor film to a reduced-pressureatmosphere, or the like. Oxygen vacancies become a source for supplyingcarriers; thus, a region where the oxygen vacancies are formed in theoxide semiconductor film affects the electrical characteristics of thetransistor. In particular, in the case where the region where oxygenvacancies are formed is located between a source and a drain, the regionserves as an unintended path of carriers, that is, a parasitic channel.A parasitic channel region generated in a side surface of the oxidesemiconductor film has a higher carrier concentration (smallerresistance) than a channel region in the oxide semiconductor film thatis intentionally formed, which causes current-voltage characteristics inwhich the threshold voltage of the parasitic channel region is shiftedin the negative direction. Thus, in the case where the parasitic channelin the side surface and the channel in the film are formed, thetransistor shows the same electrical characteristics as in the casewhere transistors are formed in parallel. A current-voltage curve thatseems to be formed by two current-voltage curves overlapping with eachother is measured. The two current-voltage curves have different valuesin the threshold voltage. As a result, the threshold voltage of thewhole transistor seems to be shifted in the negative direction. Further,leakage current flows when a voltage lower than the threshold voltage ofthe channel in the film is applied to a gate electrode.

Thus, an object of one embodiment of the present invention is to improveproductivity of a transistor that includes an oxide semiconductor havinggood electrical characteristics.

One embodiment of the present invention is a method for manufacturing atop-gate transistor that includes a gate insulating film and a gateelectrode over an oxide semiconductor film. The method includes thesteps of forming a metal film over the oxide semiconductor film, addingoxygen to the metal film to form a metal oxide film, and using the metaloxide film as the gate insulating film. After an oxide insulating filmis formed over the oxide semiconductor film, the metal film may beformed over the oxide insulating film.

Oxygen can be added to the metal film to form the metal film and addedalso to the oxide semiconductor film or the oxide insulating film thatis in contact with the metal film.

According to another embodiment of the present invention, a protectivefilm is provided over a top-gate transistor including an oxidesemiconductor film. The protective film is formed in such a manner thata metal film is formed over the transistor, oxygen is added to the metalfilm to form a metal oxide film, and the metal oxide film is used as aprotective film. After an oxide insulating film is formed over thetransistor, the metal film may be formed over the oxide insulating film.

Oxygen can be added to the metal film to form the metal oxide film andadded also to the oxide insulating film that is in contact with themetal film.

Another embodiment of the present invention is a method formanufacturing a semiconductor device including the following steps: anoxide semiconductor film that is subjected to element isolation isformed over an oxide insulating film; a metal film is formed over theoxide semiconductor film; oxygen is added to the metal film to form ametal oxide film; a gate electrode is formed over the metal oxide film;sidewall insulating films in contact with side surfaces of the gateelectrode and part of the oxide semiconductor film is exposed by etchingpart of the metal oxide film; and a pair of electrodes in contact withat least the oxide semiconductor film and the sidewall insulating filmsis formed.

After the oxide insulating film is formed over the oxide semiconductorfilm, the metal film may be formed over the oxide insulating film.

Another embodiment of the present invention is a method formanufacturing a semiconductor device including the following steps: anoxide semiconductor film that is subjected to element isolation isformed over a first oxide insulating film, a gate electrode is formedover the oxide semiconductor film, a gate insulating film is formedbetween the oxide semiconductor film and the gate electrode, sidewallinsulating films are formed in contact with side surfaces of the gateelectrode, a pair of electrodes is formed in contact with at least theoxide semiconductor film and the sidewall insulating films, a metal filmis formed over the gate electrode and the pair of electrodes, and oxygenis added to the metal film to form a metal oxide film.

After the oxide insulating film is formed over the gate electrode andthe pair of electrodes, the metal film may be formed over the oxideinsulating film.

By adding oxygen to the metal film by an ion implantation method, an iondoping method, a plasma immersion ion implantation method, plasmatreatment, or the like, the metal film is oxidized, so that a metaloxide film can be formed. The oxygen addition treatment may be performedwhile heating is performed. The method for forming a metal oxide film byadding oxygen to a metal film can suppress generation of powdercontaminants compared with a method for forming a metal oxide filmformed by deposition, so that yield can be increased and thus the methodis suitable for not only experiments but also mass production process.

Further, a metal film is formed over the oxide semiconductor film,oxygen can be added to the metal film and added also to the oxidesemiconductor film.

The metal oxide film is used as a gate insulating film of a transistorincluding an oxide semiconductor film. Since oxygen can be added to ametal film and added also to the oxide semiconductor film, oxygenvacancies in the oxide semiconductor film can be reduced.

A metal film is formed over the oxide insulating film, so that oxygencan be added to the metal film and added also to the oxide insulatingfilm or the oxide semiconductor film.

The oxide insulating film and the metal oxide film to which oxygen isadded are used as a gate insulating film of a transistor including anoxide semiconductor film. That is, the oxide insulating film in contactwith the oxide semiconductor film contains the oxygen. Further, themetal oxide film functions as an oxygen diffusion prevention film.Accordingly, oxygen in the oxide insulating film can be efficientlydiffused into the oxide semiconductor film by heat treatment so thatoxygen vacancies in the oxide semiconductor film can be reduced.

An oxide insulating film is formed over a top-gate transistor includingan oxide semiconductor film and a metal film is formed over the oxideinsulating film; after that, oxygen can be added to the metal film andadded also to the oxide insulating film. That is, the oxide insulatingfilm contains the oxygen. Further, the metal oxide film functions as anoxygen diffusion prevention film. Accordingly, the oxygen in the oxideinsulating film can be efficiently diffused into the oxide semiconductorfilm by heat treatment so that oxygen vacancies in the oxidesemiconductor film can be reduced. The metal oxide film also functionsas a film for preventing entry of hydrogen, water, and the like; thus,entry of hydrogen, water, and the like from the outside to the oxidesemiconductor film in the transistor can be suppressed. Therefore,leakage current of the transistor can be reduced.

In particular, oxygen vacancies in a side surface of the oxidesemiconductor film are compensated by diffusion of the oxygen, so thatgeneration of a parasitic channel can be suppressed. As a result,leakage current between the source electrode and the drain electrodeflowing through the side surfaces of the oxide semiconductor filmoverlapping with the gate electrode can be reduced.

With one embodiment of the present invention, a transistor having smallleakage current and having excellent electrical characteristics can bemanufactured with high productivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are a top view and cross-sectional views illustrating asemiconductor device according to one embodiment of the presentinvention.

FIGS. 2A to 2H are cross-sectional views illustrating a method formanufacturing a semiconductor device, according to one embodiment of thepresent invention.

FIGS. 3A and 3B are cross-sectional views illustrating a semiconductordevice according to one embodiment of the present invention.

FIGS. 4A to 4F are cross-sectional views illustrating a method formanufacturing a semiconductor device, according to one embodiment of thepresent invention.

FIGS. 5A and 5B are cross-sectional views illustrating a semiconductordevice according to one embodiment of the present invention.

FIGS. 6A and 6B are cross-sectional views illustrating a semiconductordevice according to one embodiment of the present invention.

FIGS. 7A to 7D are cross-sectional views illustrating a method formanufacturing a semiconductor device, according to one embodiment of thepresent invention.

FIGS. 8A and 8B are cross-sectional views illustrating a semiconductordevice according to one embodiment of the present invention.

FIGS. 9A and 9B are cross-sectional views illustrating a semiconductordevice according to one embodiment of the present invention.

FIGS. 10A to 10C are cross-sectional views illustrating a method formanufacturing a semiconductor device, according to one embodiment of thepresent invention.

FIG. 11 is a cross-sectional view illustrating a method formanufacturing a semiconductor device, according to one embodiment of thepresent invention.

FIG. 12 is a cross-sectional view illustrating a method formanufacturing a semiconductor device, according to one embodiment of thepresent invention.

FIGS. 13A to 13C are cross-sectional views illustrating a method formanufacturing a semiconductor device, according to one embodiment of thepresent invention.

FIGS. 14A to 14D are cross-sectional views illustrating a method formanufacturing a semiconductor device, according to one embodiment of thepresent invention.

FIGS. 15A to 15C are a cross-sectional view, a top view, and a circuitdiagram illustrating one embodiment of a semiconductor device.

FIGS. 16A and 16B are a circuit diagram and a perspective viewillustrating one embodiment of a semiconductor device.

FIGS. 17A and 17B are a cross-sectional view and a top view illustratingone embodiment of a semiconductor device.

FIGS. 18A and 18B are circuit diagrams each illustrating one embodimentof a semiconductor device.

FIG. 19 is a block diagram illustrating one embodiment of asemiconductor device.

FIG. 20 is a block diagram illustrating one embodiment of asemiconductor device.

FIG. 21 is a block diagram illustrating one embodiment of asemiconductor device.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention are described below with reference to theaccompanying drawings. Note that the present invention is not limited tothe description below, and it is easily understood by those skilled inthe art that various changes and modifications can be made withoutdeparting from the spirit and scope of the present invention. Therefore,the invention should not be construed as being limited to thedescription in the following embodiments. Note that the same portions orportions having the same function in the structure of the presentinvention described below are denoted by the same reference numerals incommon among different drawings and repetitive description thereof isomitted.

Note that in each drawing described in this specification, the size, thefilm thickness, or the region of each component is exaggerated forclarity in some cases. Therefore, embodiments of the present inventionare not limited to such scales.

Note that terms such as “first”, “second”, and “third” in thisspecification are used in order to avoid confusion among components, andthe terms do not limit the components numerically. Therefore, forexample, the term “first” can be replaced with the term “second”,“third”, or the like as appropriate.

(Embodiment 1)

In this embodiment, a structure of a transistor having excellentelectrical characteristics and a method for manufacturing the transistorwith high productivity are described with reference to FIGS. 1A to 1C,FIGS. 2A to 2H, FIGS. 3A and 3B, FIGS. 4A to 4F, and FIGS. 5A and 5B.

FIGS. 1A to 1C are a top view and cross-sectional views of a transistordescribed in this embodiment. FIG. 1A is a top view of the transistordescribed in this embodiment. FIG. 1B is a cross-sectional view in thechannel width direction of the transistor taken along dashed-dotted lineA-B in FIG. 1A. FIG. 1C is a cross-sectional view in the channel lengthdirection of the transistor taken along dashed-dotted line C-D in FIG.1A. Note that in FIG. 1A, illustration of some components of thetransistor (e.g., a gate insulating film 123, sidewall insulating films121, an insulating film 116, and an insulating film 127) is omitted forclarity.

The transistor illustrated in FIGS. 1A to 1C includes an oxideinsulating film 103 over a substrate 101, an oxide semiconductor film119 over the oxide insulating film 103, a pair of electrodes 125 that isin contact with the oxide semiconductor film 119 and functions as asource electrode and a drain electrode, the gate insulating film 123 incontact with at least part of the oxide semiconductor film 119, and agate electrode 115 provided over the gate insulating film 123 to overlapwith the oxide semiconductor film 119.

Further, the transistor includes the sidewall insulating films 121 incontact with side surfaces of the gate electrode 115. The oxidesemiconductor film 119 includes a first region 119 a overlapping withthe gate electrode 115 and a pair of second regions 119 b between whichthe first region 119 a is interposed and that contains dopants. Thesecond regions 119 b have a lower resistance than a channel region. Inthe oxide semiconductor film 119, the first region 119 a serves as achannel region, and regions of the pair of second regions 119 bcontaining the dopants that are in contact with the pair of electrodes125 serve as a source region and a drain region. The transistor mayinclude the insulating film 127 over the oxide insulating film 103, thegate electrode 115, the sidewall insulating films 121, and the pair ofelectrodes 125. The insulating film 116 is preferably provided betweenthe gate electrode 115 and the insulating film 127.

In the transistor illustrated in FIGS. 1A to 1C, the gate insulatingfilm 123 has a stacked-layer structure of the oxide insulating film 123a in contact with the oxide semiconductor film 119 and the metal oxidefilm 123 b in contact with the gate electrode 115. The metal oxide film123 b is formed using a metal oxide film formed by adding oxygen to ametal film.

There is no particular limitation on the property of a material and thelike of the substrate 101 as long as the material has heat resistanceenough to withstand at least heat treatment to be performed later. Forexample, a glass substrate, a ceramic substrate, a quartz substrate, asapphire substrate, or the like may be used as the substrate 101.Alternatively, a single crystal semiconductor substrate or apolycrystalline semiconductor substrate made of silicon, siliconcarbide, or the like, a compound semiconductor substrate made of silicongermanium or the like, an SOI substrate, or the like may be used as thesubstrate 101. Furthermore, any of these substrates further providedwith a semiconductor element may be used as the substrate 101.

Still further alternatively, a flexible substrate may be used as thesubstrate 101, and the oxide insulating film 103 and the transistor maybe provided directly on the flexible substrate. Alternatively, aseparation layer may be provided between the substrate 101 and the oxideinsulating film 103. The separation layer can be used when part or thewhole of a semiconductor device formed over the separation layer isseparated from the substrate 101 and transferred onto another substrate.In such a case, the semiconductor device can be transferred to asubstrate having low heat resistance or a flexible substrate as well.

The oxide insulating film 103 is preferably formed using an oxideinsulating film from which part of oxygen is released by heating. Theoxide insulating film from which part of oxygen is released by heatingis preferably an oxide insulating film that contains oxygen at aproportion exceeding the stoichiometric proportion. The oxide insulatingfilm from which part of oxygen is released by heating can diffuse oxygeninto the oxide semiconductor film by heating. Typical examples of thefirst oxide insulating film 103 are films of silicon oxide, siliconoxynitride, silicon nitride oxide, gallium oxide, hafnium oxide, yttriumoxide, aluminum oxide, aluminum oxynitride, and the like.

The thickness of the oxide insulating film 103 is greater than or equalto 50 nm, preferably greater than or equal to 200 nm and less than orequal to 3000 nm, more preferably greater than or equal to 300 nm andless than or equal to 500 nm. With use of the thick oxide insulatingfilm 103, the amount of oxygen released from the oxide insulating film103 can be increased, and the interface state at an interface betweenthe oxide insulating film 103 and an oxide semiconductor film to beformed later can be reduced.

Here, “to release part of oxygen by heating” means that the amount ofreleased oxygen is greater than or equal to 1.0×10¹⁸ atoms/cm³,preferably greater than or equal to 3.0×10²⁰ atoms/cm³ in thermaldesorption spectroscopy (TDS) on an oxygen atom basis.

Here, a method for measuring the amount of released oxygen on an oxygenatom basis using TDS analysis is described.

The amount of released gas in TDS analysis is proportional to theintegral value of a spectrum. Therefore, the amount of released gas canbe calculated from the ratio of the integral value of a spectrum of theinsulating film to the reference value of a standard sample. Thereference value of a standard sample refers to the ratio of the densityof a predetermined atom contained in a sample to the integral value of aspectrum.

For example, the number of the released oxygen molecules (N_(O2)) froman insulating film can be calculated according to Formula 1 using theTDS analysis results of a silicon wafer containing hydrogen at apredetermined density, which is the standard sample, and the TDSanalysis results of the insulating film. Here, all spectra having a massnumber of 32 that are obtained by the TDS analysis are assumed tooriginate from an oxygen molecule. CH₃OH can be given as a gas having amass number of 32, but is not taken into consideration on the assumptionthat it is unlikely to be present. Further, an oxygen molecule includingan oxygen atom having a mass number of 17 or 18, which is an isotope ofan oxygen atom, is also not taken into consideration because theproportion of such a molecule in the natural world is minimalN_(O2)=N_(H2)/S_(H2)×S_(O2)×α  (Formula 1)

N_(H2) is the value obtained by conversion of the number of hydrogenmolecules released from the standard sample into density. S_(H2) is anintegral value of a spectrum of the standard sample that is analyzed byTDS. Here, the reference value of the standard sample is set toN_(H2)/S_(H2). S_(O2) is an integral value of a spectrum when theinsulating film is analyzed by TDS. α is a coefficient affecting theintensity of the spectrum in the TDS analysis. For details of Formula 1,Japanese Published Patent Application No. H6-275697 is referred to. Notethat the amount of released oxygen from the above insulating film ismeasured with a thermal desorption spectroscopy apparatus produced byESCO Ltd., EMD-WA1000 S/W using a silicon wafer containing hydrogenatoms at 1×10¹⁶ atoms/cm² as the standard sample.

Further, in the TDS analysis, part of oxygen is detected as an oxygenatom. The ratio between oxygen molecules and oxygen atoms can becalculated from the ionization rate of oxygen molecules. Note that,since the above a includes the ionization rate of oxygen molecules, thenumber of the released oxygen atoms can also be estimated through theevaluation of the number of the released oxygen molecules.

Note that N_(O2) is the number of the released oxygen molecules. Theamount of released oxygen on an oxygen atom basis is twice the number ofthe released oxygen molecules.

In the above structure, the insulating film from which oxygen isreleased by heating may be oxygen-excess silicon oxide (SiO_(X) (X>2)).In the oxygen-excess silicon oxide (SiO_(X) (X>2)), the number of oxygenatoms per unit volume is more than twice the number of silicon atoms perunit volume. The number of silicon atoms and the number of oxygen atomsper unit volume are measured by Rutherford backscattering spectrometry.

By supplying oxygen from the oxide insulating film 103 to the oxidesemiconductor film 119, the interface state at the interface between theoxide insulating film 103 and the oxide semiconductor film 119 can bereduced. As a result, capture of electric charge that may be generateddue to operation of a transistor or the like at the interface betweenthe oxide insulating film 103 and the oxide semiconductor film 119 canbe suppressed. Thus, it is possible to provide a transistor with lesselectrical characteristic deterioration, in which negative shift of thethreshold voltage can be reduced.

Further, electric charge may be generated owing to oxygen vacancies inthe oxide semiconductor film 119 in some cases. In general, part ofoxygen vacancies in the oxide semiconductor film serves as a donor togenerate an electron that is a carrier. As a result, the thresholdvoltage of the transistor shifts in the negative direction. Thistendency is remarkable in oxygen vacancies caused on the backchannelside. Note that the term “back channel” in this specification refers tothe vicinity of an interface of the first region 119 a in the oxidesemiconductor region 119 on the oxide insulating film 103 side in FIG.1B. By supplying sufficient oxygen from the oxide insulating film 103 tothe oxide semiconductor film 119, oxygen vacancies in the oxidesemiconductor film 119 that are a cause of negative shift of thethreshold voltage can be compensated.

In other words, when oxygen vacancies are generated in the oxidesemiconductor film 119, electric charge is captured at the interfacebetween the oxide insulating film 103 and the oxide semiconductor film119, whereby the electric charge affects the electrical characteristicsof the transistor. However, by providing an insulating film from whichoxygen is released by heating as the oxide insulating film 103, theinterface state between the oxide semiconductor film 119 and the oxideinsulating film 103 and oxygen vacancies in the oxide semiconductor film119 can be reduced, and an influence of the capture of electric chargeat the interface between the oxide semiconductor film 119 and the oxideinsulating film 103 can be made small.

The oxide semiconductor film 119 preferably contains at least indium(In) or zinc (Zn). Alternatively, the oxide semiconductor film 119preferably contains both In and Zn. In order to reduce variation inelectrical characteristics of the transistor including the oxidesemiconductor film, the oxide semiconductor film 119 preferably containsone or more of stabilizers in addition to In or Zn.

As a stabilizer, gallium (Ga), tin (Sn), hafnium (Hf), aluminum (Al),zirconium (Zr), and the like can be given.

As another stabilizer, lanthanoid such as lanthanum (La), cerium (Ce),praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu),gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium(Er), thulium (Tm), ytterbium (Yb), or lutetium (Lu) can be given.

As the oxide semiconductor, for example, any of the following can beused: indium oxide; tin oxide; zinc oxide; a two-component metal oxidesuch as an In—Zn-based oxide, a Sn—Zn-based oxide, an Al—Zn-based oxide,a Zn—Mg-based oxide, a Sn—Mg-based oxide, an In—Mg-based oxide, or anIn—Ga-based oxide; a three—component metal oxide such as anIn—Ga—Zn-based oxide (also referred to as IGZO), an In—Al—Zn-basedoxide, an In—Sn—Zn-based oxide, a Sn—Ga—Zn-based oxide, anAl—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide, an In—Hf—Zn-based oxide,an In—La—Zn-based oxide, an In—Ce—Zn-based oxide, an In—Pr—Zn-basedoxide, an In—Nd—Zn-based oxide, an In—Sm—Zn-based oxide, anIn—Eu—Zn-based oxide, an In—Gd—Zn-based oxide, an In—Tb—Zn-based oxide,an In—Dy—Zn-based oxide, an In—Ho—Zn-based oxide, an In—Er—Zn-basedoxide, an In—Tm—Zn-based oxide, an In—Yb—Zn-based oxide, or anIn—Lu—Zn-based oxide; and a four—component metal oxide such as anIn—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-based oxide, anIn—Al—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, anIn—Sn—Hf—Zn-based oxide, or an In—Hf—Al—Zn-based oxide.

Note that here, for example, an “In—Ga—Zn-based oxide” means an oxidecontaining In, Ga, and Zn as its main component and there is noparticular limitation on the ratio of In:Ga:Zn. The In—Ga—Z-based oxidemay contain another metal element in addition to In, Ga, and Zn.

Alternatively, a material represented by a chemical formula,InMO₃(ZnO)_(m) (m>0, m is not an integer) may be used as an oxidesemiconductor. Note that M represents one or more metal elementsselected from Ga, Fe, Mn, and Co. Alternatively, as the oxidesemiconductor, a material expressed by a chemical formula,In₂SnO₅(ZnO)_(n) (n>0, n is an integer) may be used.

For example, an In—Ga—Zn-based oxide with an atomic ratio of In to Gaand Zn of 1:1:1 (=1/3:1/3:1/3), 2:2:1 (=2/5:2/5:1/5), or 3:1:2(=1/2:1/6:1/3), or an oxide with an atomic ratio close to the aboveatomic ratios can be used. Alternatively, an In—Sn—Zn-based oxide withan atomic ratio of In to Sn and Zn of 1:1:1 (=1/3:1/3:1/3), 2:1:3(=1/3:1/6:1/2), or 2:1:5 (=1/4:1/8:5/8), or an oxide with an atomicratio close to the above atomic ratios may be used.

However, without limitation to the materials given above, a materialwith an appropriate composition may be used depending on neededelectrical characteristics (e.g., mobility, threshold voltage, andvariation). In order to obtain the needed semiconductor characteristics,it is preferable that the carrier concentration, the impurityconcentration, the defect density, the atomic ratio between a metalelement and oxygen, the interatomic distance, the density, and the likebe set to appropriate values.

For example, high mobility can be obtained relatively easily in the caseof using an In—Sn—Zn-based oxide. However, the mobility can be increasedby reducing the defect density in the bulk also in the case of using theIn—Ga—Zn-based oxide.

Further, the energy gap of a metal oxide that can form the oxidesemiconductor film 119 is greater than or equal to 2 eV, preferablygreater than or equal to 2.5 eV, more preferably greater than or equalto 3 eV. In this manner, the off-state current of a transistor can bereduced by using an oxide semiconductor having a wide energy gap.

Note that the oxide semiconductor film 119 may have an amorphousstructure, a single crystal structure, or a polycrystalline structure.

As the oxide semiconductor film 119, a c-axis aligned crystalline oxidesemiconductor film (also referred to as CAAC-OS film) having crystalparts may be used.

The CAAC-OS film is neither absolutely single crystal nor absolutelyamorphous. The CAAC-OS film is an oxide semiconductor film with acrystal-amorphous mixed phase structure where crystal parts andamorphous parts are included in an amorphous phase. Note that in mostcases, the crystal part fits inside a cube whose one side is less than100 nm. Note that from an observation image obtained with a transmissionelectron microscope (TEM), a boundary between an amorphous part and acrystal part in the CAAC-OS film is not always clear. Further, with theTEM, a grain boundary in the CAAC-OS film is not found. Thus, in theCAAC-OS film, a reduction in electron mobility, due to the grainboundary, is suppressed.

In each of the crystal parts included in the CAAC-OS film, a c-axis isaligned in a direction parallel to a normal vector of a surface wherethe CAAC-OS film is formed or a normal vector of a surface of theCAAC-OS film, triangular or hexagonal atomic arrangement that is seenfrom the direction perpendicular to the a-b plane is formed, and metalatoms are arranged in a layered manner or metal atoms and oxygen atomsare arranged in a layered manner when seen from the directionperpendicular to the c-axis. Note that, among crystal parts, thedirections of the a-axis and the b-axis of one crystal part may bedifferent from those of another crystal part. In this specification, asimple term “perpendicular” includes a range from 85° to 95°. Inaddition, a simple term “parallel” includes a range from −5° to 5°. Notethat part of oxygen included in the oxide semiconductor film may besubstituted with nitrogen.

In the CAAC-OS film, distribution of crystal parts is not necessarilyuniform. For example, in the formation process of the CAAC-OS film, inthe case where crystal growth occurs from a surface side of the oxidesemiconductor film, the proportion of crystal parts in the vicinity ofthe surface of the oxide semiconductor film is higher than that in thevicinity of the surface where the oxide semiconductor film is formed insome cases. Further, when impurities are added to the CAAC-OS film, thecrystal part in a region to which the impurities are added becomesamorphous in some cases.

Since the c-axes of the crystal parts included in the CAAC-OS film arealigned in the direction parallel to a normal vector of a surface wherethe CAAC-OS film is formed or a normal vector of a surface of theCAAC-OS film, the directions of the c-axes may be different from eachother depending on the shape of the CAAC-OS film (the cross-sectionalshape of the surface where the CAAC-OS film is formed or thecross-sectional shape of the surface of the CAAC-OS film). Note thatwhen the CAAC-OS film is formed, the direction of c-axis of the crystalpart is the direction parallel to a normal vector of the surface wherethe CAAC-OS film is formed or a normal vector of the surface of theCAAC-OS film. The crystal part is formed by film formation or byperforming treatment for crystallization such as heat treatment afterfilm formation.

With use of the CAAC-OS film in a transistor, change in electricalcharacteristics of the transistor due to irradiation with visible lightor ultraviolet light is small. Thus, the transistor has highreliability.

Further, the oxide semiconductor film 119 may have a structure in whicha plurality of oxide semiconductor films is stacked. For example, theoxide semiconductor film 119 may have a stacked-layer structure of afirst oxide semiconductor film and a second oxide semiconductor filmthat are formed using metal oxides with different compositions. Forexample, the first oxide semiconductor film may be formed using any of atwo-component metal oxide, a three-component metal oxide, and afour-component metal oxide, and the second oxide semiconductor film maybe formed using any of these that is different from the oxide for thefirst oxide semiconductor film.

Further, the constituent elements of the first oxide semiconductor filmand the second oxide semiconductor film are made to be the same and thecomposition of the constituent elements of the first oxide semiconductorfilm and the second oxide semiconductor film may be made to bedifferent. For example, the first oxide semiconductor film may have anatomic ratio of In to Ga and Zn of 1:1:1 and the second oxidesemiconductor film may have an atomic ratio of In to Ga and Zn of 3:1:2.Alternatively, the first oxide semiconductor film may each have anatomic ratio of In to Ga and Zn of 1:3:2, and the second oxidesemiconductor film may have an atomic ratio of In to Ga and Zn of 2:1:3.

At this time, one of the first oxide semiconductor film and the secondoxide semiconductor film that is closer to a gate electrode (on achannel side) preferably contains In and Ga at a proportion of In>Ga.The other that is farther from the gate electrode (on a back channelside) preferably contains In and Ga at a proportion of In Ga.

In an oxide semiconductor, the s orbital of heavy metal mainlycontributes to carrier transfer, and when the In content in the oxidesemiconductor is increased, overlap of the s orbital is likely to beincreased. Therefore, an oxide having a composition of In>Ga has highermobility than an oxide having a composition of In≦Ga. Further, in Ga,the formation energy of oxygen vacancies is larger and thus oxygenvacancies are less likely to occur, than in In; therefore, the oxidehaving a composition of In≦Ga has more stable characteristics than theoxide having a composition of In>Ga.

An oxide semiconductor containing In and Ga at a proportion of In>Ga isused on a channel side, and an oxide semiconductor containing In and Gaat a proportion of In≦Ga is used on a back channel side, so thatfield-effect mobility and reliability of a transistor can be furtherimproved.

Oxide semiconductors that differ in crystallinity may be applied to thefirst oxide semiconductor film and the second oxide semiconductor film.That is, the first oxide semiconductor film and the second oxidesemiconductor film may each be formed by combining a single crystaloxide semiconductor, a polycrystalline oxide semiconductor, an amorphousoxide semiconductor, and a CAAC-OS as appropriate. An amorphous oxidesemiconductor is applied to at least one of the first oxidesemiconductor film and the second oxide semiconductor film, so thatinternal stress or external stress of the oxide semiconductor film 119is relieved, variation in characteristics of a transistor is reduced,and reliability of the transistor can be further improved.

On the other hand, an amorphous oxide semiconductor tends to become ann-type (have a low resistance) because the amorphous oxide semiconductortends to absorb impurities serving as donor, such as hydrogen and water,and oxygen vacancies tend to be formed in the amorphous oxidesemiconductor by hydrogen. Accordingly, an oxide semiconductor havingcrystallinity, such as a CAAC-OS, is preferably applied to the oxidesemiconductor film on the channel side.

The thickness of the oxide semiconductor film 119 is greater than orequal to 1 nm and less than or equal to 50 nm, preferably greater thanor equal to 1 nm and less than or equal to 30 nm, more preferablygreater than or equal to 1 nm and less than or equal to 10 nm, stillmore preferably greater than or equal to 3 nm and less than or equal to7 nm. The thickness of the oxide semiconductor film 119 is in the aboverange, whereby the shift of the threshold voltage of the transistor inthe negative direction can be suppressed in the case where the channellength of the transistor is reduced.

The concentration of alkali metals or alkaline earth metals in the oxidesemiconductor film 119 is preferably lower than or equal to 1×10¹⁸atoms/cm³, more preferably lower than or equal to 2×10¹⁶ atoms/cm³. Whenalkali metals or alkaline earth metals are bonded to an oxidesemiconductor, some of the alkali metals or the alkaline earth metalsgenerate carriers and cause an increase in the off-state current of thetransistor.

Nitrogen may be contained in the first region 119 a in the oxidesemiconductor film 119 at a concentration of lower than or equal to5×10¹⁸ atoms/cm³.

Further, the concentration of hydrogen in the first region 119 a in theoxide semiconductor film 119 is preferably lower than 5×10¹⁸ atoms/cm³,more preferably lower than or equal to 1×10¹⁸ atoms/cm³, still morepreferably lower than or equal to 5×10¹⁷ atoms/cm³, still morepreferably lower than or equal to 1×10¹⁶ atoms/cm³. By a bond of anoxide semiconductor and hydrogen, part of contained hydrogen serves as adonor to generate electrons as carriers. For that reason, by a reductionin the concentration of hydrogen in the first region 119 a in the oxidesemiconductor film 119, a negative shift of the threshold voltage can bereduced.

The pair of second regions 119 b contains, as dopants, at least one ofboron, nitrogen, phosphorus, and arsenic. Alternatively, the pair ofsecond regions 119 b contains, as dopants, at least one of helium, neon,argon, krypton, and xenon. Still alternatively, the pair of secondregions 119 b may contain, as dopants, at least one of boron, nitrogen,phosphorus, and arsenic and at least one of helium, neon, argon,krypton, and xenon in appropriate combination.

The concentration of the dopants contained in the pair of second regions119 b is higher than or equal to 5×10¹⁸ atoms/cm³ and lower than orequal to 1×10²² atoms/cm³, preferably higher than or equal to 5×10¹⁸atoms/cm³ and lower than 5×10¹⁹ atoms/cm³.

Since the pair of second regions 119 b contains the dopants, the carrierdensity or the number of defects can be increased. Therefore, theconductivity can be higher than that of the first region 119 a that doesnot contain the dopants. An excessive increase in the dopantconcentration causes inhibition of carrier movement by the dopants,which leads to lower conductivity of the pair of second regions 119 bcontaining the dopants.

The pair of second regions 119 b containing the dopants preferably has aconductivity higher than or equal to 0.1 S/cm and lower than or equal to1000 S/cm, preferably higher than or equal to 10 S/cm and lower than orequal to 1000 S/cm. The source-drain breakdown voltage can be increasedby providing the second regions 119 b.

In the oxide semiconductor film 119, the first region 119 a and thesecond regions 119 b may have the same crystal structure. For example,the first region 119 a and the second regions 119 b may have a singlecrystal structure, a polycrystalline structure, or an amorphousstructure. Further, the first region 119 a and the second regions 119 bmay be formed using a CAAC-OS film.

Alternatively, in the oxide semiconductor film 119, the first region 119a and the second regions 119 b may have different crystal structures.For example, the first region 119 a may have a single crystal structureor a polycrystalline structure, and the second regions 119 b may have anamorphous structure. Alternatively, the first region 119 a may include aCAAC-OS film, and the second regions 119 b may have an amorphousstructure. Hydrogen is easily diffused into an oxide semiconductorhaving an amorphous structure; thus, hydrogen in the first region 119 ais diffused into the second regions 119 b, so that the hydrogenconcentration in the first region 119 a, which is to be a channelregion, can be reduced and the second regions 119 b become an n-type(have a low resistance).

As illustrated in FIGS. 1A and 1C, the contact area between the pair ofelectrodes 125 and the oxide semiconductor film 119 can be increased inthe case where the pair of electrodes 125 cover an exposed region and aside surface of the oxide semiconductor film 119, in particular, a sidesurface parallel with the channel length direction and a side surfaceparallel with the channel width direction. Accordingly, the contactresistance between the oxide semiconductor film 119 and the pair ofelectrodes 125 can be reduced, the channel width can be increased, andthe on-state current of the transistor can be increased.

The gate insulating film 123 has a stacked-layer structure of the oxideinsulating film 123 a in contact with the oxide semiconductor film 119and the metal oxide film 123 b in contact with the gate electrode 115.The metal oxide film 123 b is formed using a metal oxide film formed byadding oxygen to a metal film.

The oxide insulating film 123 a may be formed to have a stacked-layerstructure or a single-layer structure using, for example, silicon oxide,silicon oxynitride, and a Ga—Zn-based metal oxide.

The oxide insulating film 123 a may be an oxide insulating film fromwhich oxygen is released by heating, such as a film applicable to theoxide insulating film 103. By using a film from which oxygen is releasedby heating as the gate insulating film 123, oxygen vacancies caused inthe oxide semiconductor film 119 can be compensated by heat treatmentperformed later and deterioration of electrical characteristics of thetransistor can be suppressed.

The metal oxide film 123 b may be formed to have a stacked-layerstructure or a single-layer structure using, for example, aluminumoxide, aluminum oxynitride, gallium oxide, gallium oxynitride, yttriumoxide, yttrium oxynitride, hafnium oxide, and hafnium oxynitride.

The metal oxide film has a blocking effect against oxygen, hydrogen,water, and the like; thus, the diffusion of oxygen contained in theoxide insulating film 103, the oxide semiconductor film 119, and theoxide insulating film 123 a into the outside can be suppressed.

In the case where a high-k material such as hafnium oxide, hafniumoxynitride, yttrium oxide, or yttrium oxynitride is used for the metaloxide film 123 b, gate leakage of the transistor can be reduced.

The thickness of the gate insulating film 123 is preferably greater thanor equal to 5 nm and less than or equal to 300 nm, more preferablygreater than or equal to 10 nm and less than or equal to 50 nm, stillmore preferably greater than or equal to 10 nm and less than or equal to30 nm.

The gate electrode 115 can be formed using a metal element selected fromaluminum, chromium, copper, tantalum, titanium, molybdenum, andtungsten; an alloy containing any of these metal elements as acomponent; an alloy film containing these metal elements in combination;or the like. Further, one or more metal elements selected from manganeseor zirconium may be used. Furthermore, the gate electrode 115 may have asingle-layer structure or a stacked-layer structure of two or morelayers. For example, a single-layer structure of an aluminum filmcontaining silicon, a two-layer structure in which a titanium film isstacked over an aluminum film, a two-layer structure in which a titaniumfilm is stacked over a titanium nitride film, a two-layer structure inwhich a tungsten film is stacked over a titanium nitride film, atwo-layer structure in which a tungsten film is stacked over a tantalumnitride film or a tungsten nitride film, a three-layer structure inwhich a titanium film, an aluminum film, and a titanium film are stackedin this order, and the like can be given. Alternatively, a film, analloy film, or a nitride film that contains aluminum and one or moreelements selected from titanium, tantalum, tungsten, molybdenum,chromium, neodymium, and scandium may be used.

The gate electrode 115 can also be formed using a light-transmittingconductive material such as indium tin oxide, indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium zinc oxide, or indium tin oxide to which silicon oxide isadded. It is also possible to have a stacked-layer structure formedusing the above light-transmitting conductive material and the abovemetal element.

Further, an In—Ga—Zn—O film containing nitrogen, an In—Sn—O filmcontaining nitrogen, an In—Ga—O film containing nitrogen, an In—Zn—Ofilm containing nitrogen, a Sn—O film containing nitrogen, an In—O filmcontaining nitrogen, a film of a metal nitride (such as InN or ZnN), orthe like is preferably provided between the gate electrode 115 and thegate insulating film 123. These films each have a work function of 5 eVor higher, preferably 5.5 eV or higher; thus, the threshold voltage ofthe electrical characteristics of the transistor can be positive.Accordingly, a so-called normally-off switching element can be obtained.For example, in the case of using an In—Ga—Zn—O film containingnitrogen, an In—Ga—Zn—O film having a nitrogen concentration higher thanat least the oxide semiconductor film 119, specifically, an In—Ga—Zn—Ofilm having a nitrogen concentration of 7 at. % or higher is used.

The insulating film 116 is preferably provided over the gate electrode115 in order to prevent contact between the gate electrode 115 and thepair of electrodes 125. The insulating film 116 may each be formed witha single layer or a stack including one or more of silicon oxide,silicon oxynitride, silicon nitride oxide, silicon nitride, aluminumoxide, aluminum oxynitride, aluminum nitride oxide, aluminum nitride,and the like. By selecting an insulating film having a lower etchingrate than the sidewall insulating films 121, the insulating film canfunction as an etching protective film for reducing a reduction inthickness of the gate electrode 115 when the sidewall insulating films121 are formed, which is described later.

The sidewall insulating films 121 may each be formed with a single layeror a stack using one or more of silicon oxide, silicon oxynitride,silicon nitride oxide, silicon nitride, aluminum oxide, aluminumoxynitride, aluminum nitride oxide, aluminum nitride, and the like, forexample. The sidewall insulating films 121 may be formed using an oxideinsulating film from which part of oxygen is released by heating in amanner similar to that of the first oxide insulating film 103.

The pair of electrodes 125 is formed to have a single-layer structure ora stacked-layer structure including, as a conductive material, any ofmetals such as aluminum, titanium, chromium, nickel, copper, yttrium,zirconium, molybdenum, silver, tantalum, and tungsten and an alloycontaining any of these metals as a main component. For example, asingle-layer structure of an aluminum film containing silicon, atwo-layer structure in which a titanium film is stacked over an aluminumfilm, a two-layer structure in which a titanium film is stacked over atungsten film, a two-layer structure in which a copper film is formedover a copper-magnesium-aluminum alloy film, a three-layer structure inwhich a titanium film or a titanium nitride film, an aluminum film or acopper film, and a titanium film or a titanium nitride film are stackedin this order, a three-layer structure in which a molybdenum film or amolybdenum nitride film, an aluminum film or a copper film, and amolybdenum film or a molybdenum nitride film are stacked in this order,and the like can be given. Note that a transparent conductive materialcontaining indium oxide, tin oxide, or zinc oxide may be used. Note thatthe pair of electrodes 125 may also function as a wiring.

End portions of the pair of electrodes 125 in the transistor arepositioned over the sidewall insulating films 121, and the pair ofelectrodes 125 completely covers an exposed portion of the pair ofsecond regions 119 b containing the dopants in the oxide semiconductorfilm 119. Thus, in the channel length direction, the distance betweenthe source and the drain (more precisely, the distance between the oxidesemiconductor in contact with the source electrode and the oxidesemiconductor in contact with the drain electrode) can be controlled bythe lengths of the sidewall insulating films 121. That is, in a minutedevice in which patterning using a mask is difficult, end portions onthe channel side of the pair of electrodes 125 in contact with the oxidesemiconductor film 119 can be formed without a mask. Further, since amask is not used, variation of a plurality of transistors due toprocessing can be reduced.

The insulating film 127 may be formed with a single layer or a stackincluding one or more of silicon oxide, silicon oxynitride, siliconnitride oxide, silicon nitride, aluminum oxide, aluminum oxynitride,aluminum nitride oxide, aluminum nitride, and the like. The insulatingfilm 127 may have a stacked-layer structure, and an insulating film incontact with the pair of electrodes may be formed using an oxideinsulating film from which part of oxygen is released by heating in amanner similar to that of the insulating film 103. As the insulatingfilm on the side in contact with the pair of electrodes, an insulatingfilm that can prevent diffusion of oxygen into the outside, such as analuminum oxide film or an aluminum oxynitride film, so that oxygenreleased from the insulating film on the side in contact with the pairof electrodes can be supplied to the oxide semiconductor film. Inaddition, by using an oxide insulating film that prevents entry ofhydrogen, water, and the like from the outside as the insulating film127, entry of hydrogen, water, and the like from the outside to theoxide semiconductor film can be reduced, and oxygen vacancies in theoxide semiconductor film can be reduced. Typical examples of the oxideinsulating film that prevents entry of hydrogen, water, and the likefrom the outside are films of silicon nitride, silicon nitride oxide,aluminum nitride, aluminum nitride oxide, aluminum oxide, aluminumoxynitride, and the like.

Although a transistor in which the top shape of opposite surfaces of thepair of electrodes 125 is linear has been used in this embodiment, thetop shape of the opposite surfaces of the pair of electrodes 125 may beU-shaped or C-shaped as appropriate, for example. A transistor with sucha structure can have an increased channel width; accordingly, theon-state current can be increased.

Next, a method for manufacturing the transistor in FIGS. 1A to 1C isdescribed with reference to FIGS. 2A to 2H and FIGS. 4A to 4F. Note thatmanufacturing steps of cross section A-B (in the channel width directionof the transistor) in FIG. 1B are illustrated in FIGS. 2A, 2C, 2E, and2G and FIGS. 4A, 4C, and 4E, whereas manufacturing steps of crosssection C-D (in the channel length direction of the transistor) in FIG.1C are illustrated in FIGS. 2B, 2D, 2F, and 2H and FIGS. 4B, 4D, and 4F.

As illustrated in FIGS. 2A and 2B, the oxide insulating film 103 isformed over the substrate 101.

Note that before the oxide insulating film 103 is formed, hydrogen orwater contained in the substrate is preferably released by heattreatment or plasma treatment. Consequently, in heat treatment performedlater, diffusion of hydrogen or water into the oxide insulating film andthe oxide semiconductor film can be prevented. The heat treatment isperformed at a temperature of higher than or equal to 100° C. and lowerthan the strain point of the substrate under an inert atmosphere, areduced-pressure atmosphere, or a dry air atmosphere. Further, for theplasma treatment, rare gas, oxygen, nitrogen, or nitrogen oxide (e.g.,nitrous oxide, nitrogen monoxide, or nitrogen dioxide) is used.

The oxide insulating film 103 is formed by a sputtering method, a CVDmethod, or the like.

When the oxide insulating film from which part of oxygen is released byheating is formed by a sputtering method, the amount of oxygen in adeposition gas is preferably large, and oxygen, a mixed gas of oxygenand a rare gas, or the like can be used. Typically, the oxygenconcentration of a deposition gas is preferably from 6% to 100%.

In the case where a silicon oxide film is formed as a typical example ofan oxide insulating film from which part of oxygen is released byheating, the silicon oxide film is preferably formed by an RF sputteringmethod under the following conditions: quartz (preferably syntheticquartz) is used as a target; the substrate temperature is from 30° C. to450° C. (preferably from 70° C. to 200° C.); the distance between thesubstrate and the target (the T-S distance) is from 20 mm to 400 mm(preferably from 40 mm to 200 mm); the pressure is from 0.1 Pa to 4 Pa(preferably from 0.2 Pa to 1.2 Pa), the high-frequency power is from 0.5kW to 12 kW (preferably from 1 kW to 5 kW); and the proportion of oxygenin the deposition gas (O₂/(O₂+Ar)) is from 1% to 100% (preferably from6% to 100%). Note that a silicon target may be used as the targetinstead of the quartz (preferably synthetic quartz) target. In addition,oxygen alone may be used as the deposition gas.

In the case where an oxide insulating film is formed by a CVD method asthe oxide insulating film 103, hydrogen or water derived from a sourcegas is sometimes mixed in the oxide insulating film. Thus, after theoxide insulating film is formed by a CVD method, heat treatment ispreferably performed as dehydrogenation or dehydration.

The temperature of the heat treatment is preferably a temperature atwhich hydrogen or water is released from the oxide insulating film.Typically, the temperature is higher than or equal to 150° C. and lowerthan the strain point of the substrate, preferably higher than or equalto 250° C. and lower than or equal to 450° C., more preferably higherthan or equal to 300° C. and lower than or equal to 450° C.

An electric furnace, a rapid thermal annealing (RTA) apparatus, or thelike can be used for the heat treatment. With use of an RTA apparatus,heat treatment at a temperature higher than or equal to the strain pointof the substrate can be performed only for a short time. Thus, timeduring which hydrogen or water is released from the oxide insulatingfilm can be shortened.

The heat treatment may be performed under an atmosphere of nitrogen,oxygen, ultra-dry air (air in which the water content is 20 ppm orlower, preferably 1 ppm or lower, more preferably 10 ppb or lower), or arare gas (argon, helium, or the like). It is preferable that water,hydrogen, or the like is not contained in the atmosphere of nitrogen,oxygen, ultra-dry air, or a rare gas. It is also preferable that thepurity of nitrogen, oxygen, or the rare gas that is introduced into aheat treatment apparatus be set to be 6N (99.9999%) or higher,preferably 7N (99.99999%) or higher (that is, the impurity concentrationis 1 ppm or lower, preferably 0.1 ppm or lower). Note that the heattreatment may be performed in vacuum.

By the heat treatment, dehydrogenation or dehydration can be performedon the oxide insulating film and thus, diffusion of hydrogen or water tothe oxide semiconductor film can be suppressed.

In the case of adding oxygen to the oxide insulating film formed by aCVD method, the amount of oxygen released by heating can be increased.As the method for adding oxygen to the oxide insulating film, an ionimplantation method, an ion doping method, a plasma immersion ionimplantation method, plasma treatment, or the like can be used.

The heat treatment for dehydration or dehydrogenation may be performedplural times, and may also serve as another heat treatment.

The surface of the oxide insulating film 103 is preferably flat becausethe oxide semiconductor film formed later can be prevented from beingdisconnected.

Next, an oxide semiconductor film 105 is formed over the oxideinsulating film 103.

The oxide semiconductor film 105 can be formed as follows. An oxidesemiconductor film is formed over the oxide insulating film 103 by asputtering method, a coating method, a pulsed laser deposition method, alaser ablation method, or the like. A mask is formed over the oxidesemiconductor film. Part of the oxide semiconductor film is etched withuse of the mask, so that the oxide semiconductor film 105 that issubjected to element isolation can be formed. Alternatively, by using aprinting method for forming the oxide semiconductor film 105, the oxidesemiconductor film 105 that is subjected to element isolation can beformed directly.

Here, the oxide semiconductor film 105 is formed by a sputtering methodin such a manner that an oxide semiconductor film with a thickness ofgreater than or equal to 1 nm and less than or equal to 50 nm,preferably greater than or equal to 3 nm and less than or equal to 30 nmis formed, a mask is formed over the oxide semiconductor film, and partof the oxide semiconductor film is selectively etched.

For example, in the case where the oxide semiconductor film is formed bya sputtering method at a substrate temperature higher than or equal to150° C. and lower than or equal to 750° C., preferably higher than orequal to 150° C. and lower than or equal to 450° C., more preferablyhigher than or equal to 200° C. and lower than or equal to 350° C., theamount of hydrogen or water entering the oxide semiconductor film can bereduced and the oxide semiconductor film can be a CAAC OS film.

In order to improve the orientation of the crystal parts in the CAAC-OSfilm, planarity of the surface of the oxide insulating film 103 servingas a base insulating film in the oxide semiconductor film is preferablyimproved. Typically, an average surface roughness (R_(a)) of the oxideinsulating film 103 is preferably greater than or equal to 0.1 nm andless than 0.5 nm. In this specification and the like, average surfaceroughness (R_(a)) is obtained by three-dimension expansion of centerline average roughness (R_(a)) that is defined by JISB0601:2001 (ISO4287:1997) so that R_(a) can be applied to a curved surface, and can beexpressed as an average value of the absolute values of deviations froma reference surface to a specific surface. As planarization treatment,one or more can be selected from chemical mechanical polishing (CMP)treatment, dry etching treatment, plasma treatment (reverse sputtering),and the like. The plasma treatment is the one in which minute unevennessof the surface is reduced by introducing an inert gas such as an argongas into a vacuum chamber and applying an electric field so that asurface to be processed serves as a cathode.

Here, a sputtering apparatus used for forming the oxide semiconductorfilm is described in detail below.

The leakage rate of a treatment chamber in which the oxide semiconductorfilm is formed is preferably lower than or equal to 1×10⁻¹° Pa·m³/sec.,whereby entry of impurities into the film formed by a sputtering methodcan be decreased.

Evacuation of the treatment chamber in the sputtering apparatus ispreferably performed with a rough vacuum pump such as a dry pump and ahigh vacuum pump such as a sputter ion pump, a turbo molecular pump, ora cryopump in appropriate combination. The turbo molecular pump has anoutstanding capability in evacuating a large-sized molecule, whereas ithas a low capability in evacuating hydrogen and water. Further, acombination with a sputter ion pump having a high capability inevacuating hydrogen or a cryopump having a high capability in evacuatingwater or is effective.

An adsorbate present on the inner wall of the treatment chamber does notaffect the pressure in the treatment chamber because it is adsorbed onthe inner wall, but the adsorbate leads to release of gas at the time ofthe evacuation of the treatment chamber. Therefore, although the leakagerate and the evacuation rate do not have a correlation, it is importantthat the adsorbate present in the treatment chamber be desorbed as muchas possible and evacuation be performed in advance with use of a pumphaving high evacuation capability. Note that the treatment chamber maybe subjected to baking for promoting desorption of the adsorbate. By thebaking, the rate of desorption of the adsorbate can be increased abouttenfold. The baking may be performed at a temperature higher than orequal to 100° C. and lower than or equal to 450° C. At this time, whenthe adsorbate is removed while an inert gas is introduced, the rate ofdesorption of water or the like, which is difficult to desorb only byevacuation, can be further increased.

As described above, in the process for forming the oxide semiconductorfilm and preferably in the process for forming the oxide insulatingfilm, entry of impurities is suppressed as much as possible throughcontrol of the pressure of the treatment chamber, leakage rate of thetreatment chamber, and the like, whereby entry of impurities includinghydrogen contained in the oxide semiconductor film can be reduced. Inaddition, diffusion of impurities such as hydrogen and water from theoxide insulating film to the oxide semiconductor film can be reduced.

Hydrogen contained in the oxide semiconductor reacts with oxygen bondedto a metal atom to produce water, and a defect is formed in a latticefrom which oxygen is released (or a portion from which oxygen isremoved). Thus, the impurities containing hydrogen are reduced as muchas possible in the formation step of the oxide semiconductor film,whereby defects in the oxide semiconductor film can be reduced.Therefore, when a channel region is formed in an oxide semiconductorfilm that is purified by removing impurities as much as possible, thetransistor can have higher reliability.

A power supply device for generating plasma in a sputtering method canbe an RF power supply device, an AC power supply device, a DC powersupply device, or the like as appropriate.

As a sputtering gas, a rare gas (typically argon) atmosphere, an oxygenatmosphere, or a mixed gas of a rare gas and oxygen is used asappropriate. In the case of using the mixed gas of a rare gas andoxygen, the proportion of oxygen is preferably higher than that of arare gas. It is preferable that a high-purity gas from which impuritiescontaining hydrogen are removed be used as a sputtering gas.

Note that before the oxide semiconductor film is formed by a sputteringapparatus, a dummy substrate may be put into the sputtering apparatus,and an oxide semiconductor film may be formed over the dummy substrate,so that hydrogen, water, and the like attached to the target surface ora deposition shield may be removed.

By forming the oxide insulating film 103 and the oxide semiconductorfilm successively without exposure to the air, impurities such ashydrogen and water in the air can be prevented from entering theinterface between the oxide insulating film 103 and the oxidesemiconductor film, which is preferable. For example, the oxideinsulating film 103 is formed in a first treatment chamber of amulti-chamber sputtering apparatus. Next, the substrate 101 over whichthe oxide insulating film 103 is formed is heated in a preheatingchamber, and impurities such as hydrogen and water contained in thesubstrate 101 and the oxide insulating film 103 are released. Note thatthe heating temperature at this time is preferably in a temperaturerange in which oxygen is not released form the oxide insulating film103. Next, the oxide semiconductor film is formed in a second treatmentchamber, so that the oxide insulating film and the oxide semiconductorfilm can be formed successively without exposure to the air.

In this case, a glass substrate is used as the substrate. First, in apreheating chamber of a multi-chamber sputtering apparatus, thesubstrate is heated so that moisture or the like contained in thesubstrate is released. Next, in a first treatment chamber, a siliconoxide film with a thickness of 300 nm is formed as the oxide insulatingfilm 103 without exposure to the air. Then, in a second treatmentchamber, an oxide semiconductor film (In—Ga—Zn-based oxide film) with athickness of 20 nm is formed. Subsequently, a mask is formed over theoxide semiconductor film by a photolithography process, and the oxidesemiconductor film is dry-etched with use of the mask to form the oxidesemiconductor film 105.

Next, heat treatment is preferably performed on the substrate 101. Bythe heat treatment, dehydrogenation or dehydration of the oxidesemiconductor film 105 can be performed.

Further, part of the oxygen contained in the oxide insulating film 103can be diffused into the oxide semiconductor film 105 and a vicinity ofthe interface between the oxide insulating film 103 and the oxidesemiconductor film 105.

The temperature of the heat treatment is typically higher than or equalto 150° C. and lower than the strain point of the substrate, preferablyhigher than or equal to 250° C. and lower than or equal to 450° C., morepreferably higher than or equal to 300° C. and lower than or equal to450° C.

An electric furnace, an RTA apparatus, or the like can be used for theheat treatment. With use of an RTA apparatus, heat treatment at atemperature higher than or equal to the strain point of the substratecan be performed only for a short time. Thus, the time during whichhydrogen or water is released from the oxide semiconductor film and thetime during which oxygen is diffused from the oxide insulating film 103into the oxide semiconductor film 105 can be shortened.

The heat treatment is performed under an inert gas atmosphere containingnitrogen or a rare gas such as helium, neon, argon, xenon, or krypton.Alternatively, the heat treatment may be performed under an inert gasatmosphere first, and then under an oxygen atmosphere. It is preferablethat the above inert gas atmosphere and the above oxygen atmosphere donot contain hydrogen, water, and the like. The treatment time is 3minutes to 24 hours.

The oxide semiconductor film 105 may be formed as follows. An oxidesemiconductor film is formed over the oxide insulating film 103, and theheat treatment for dehydrogenation or dehydration is performed. Afterthat, part of the oxide semiconductor film is etched to form the oxidesemiconductor film 105 that is subjected to element isolation. With suchsteps, since the oxide insulating film 103 is completely covered withthe oxide semiconductor film in the heat treatment for dehydrogenationor dehydration, the oxygen contained in the oxide insulating film 103can be efficiently diffused into the oxide semiconductor film.

The heat treatment for dehydrogenation or dehydration may be performedafter an oxide insulating film 107 to be formed later is formed. As aresult, the number of the heat treatment steps can be reduced and wateror hydrogen can be released from the oxide semiconductor film 105 andthe oxide insulating film 107.

The heat treatment for dehydration or dehydrogenation may be performedplural times, and may also serve as another heat treatment.

Next, as illustrated in FIGS. 2C and 2D, after the oxide insulating film107 is formed over the oxide insulating film 103 and the oxidesemiconductor film 105, a metal film 109 is formed over the oxideinsulating film 107.

Since the oxide insulating film 107 becomes the oxide insulating film123 a later, the materials given for the oxide insulating film 123 a inFIGS. 1A to 1C can be used as appropriate. The oxide insulating film 107is formed by a sputtering method, a CVD method, or the like.

Since the oxide insulating film 107 is formed before the metal film 109is formed, the oxide semiconductor film 105 is not directly in contactwith the metal film 109, so that reaction of the oxide semiconductorfilm 105 to the metal film can be reduced. Accordingly, quality changeof a region to be a channel region later in the oxide semiconductor film105 can be prevented.

As the metal film 109, a metal film having a blocking effect againstoxygen, hydrogen, water, and the like in the state of being oxidized ispreferably used. Typical examples thereof are aluminum, gallium,yttrium, and hafnium. The metal film 109 is formed by a sputteringmethod, an evaporation method, or the like.

The thickness of the metal film 109 is preferably greater than or equalto 5 nm and less than 30 nm, more preferably greater than or equal to 10nm and less than or equal to 20 nm. By setting the thickness of themetal film 109 to the above thickness, in later oxygen-adding treatment,the metal film 109 can be oxidized and oxygen can be added to the oxideinsulating film 107.

Next, oxygen 111 is added to the metal film 109 to oxidize the metalfilm 109 and oxygen is added to the oxide insulating film 107, so thatan oxide insulating film 112 and a metal oxide film 113 to which oxygenis added are formed as illustrated in FIGS. 2E and 2F. At this time, themetal oxide film 113 is a metal oxide film or a metal oxynitride filmcorresponding to the metal film 109. For example, in the case wherealuminum is used for the metal film 109, an aluminum oxide film or analuminum oxynitride film is formed as the metal oxide film 113.

The oxide insulating film 112 to which oxygen is added is preferably anoxide insulating film containing oxygen satisfying the stoichiometricproportion or an oxide insulating film containing oxygen that is morethan oxygen satisfying the stoichiometric proportion.

The metal oxide film 113 is preferably a metal oxide film containingoxygen satisfying the stoichiometric proportion or a metal oxide filmcontaining oxygen that is more than oxygen satisfying the stoichiometricproportion.

As a method for adding oxygen to the oxide insulating film 107 and themetal film 109, an ion implantation method, an ion doping method, aplasma immersion ion implantation method, plasma treatment, or the likecan be used. Note that for the ion implantation method, a gas clusterion beam may be used. The oxygen 111 may be added to the entire area ofthe substrate 101 at a time. Alternatively, a linear ion beam may beused for adding the oxygen. In the case of using a linear ion beam,relative movement (scanning) of the substrate or the ion beam enablesthe oxygen 111 to be added to the entire surfaces of the oxideinsulating film 107 and the metal film 109. The oxygen-adding treatmentmay be performed while heat treatment is performed.

Typical examples of the oxygen 111 added to the oxide insulating film107 and the metal film 109 are an oxygen radical, ozone, an oxygen atom,and an oxygen ion. The oxygen 111 can be generated by a gas containingoxygen. Typical examples of the gas containing oxygen are an oxygen gas,a dinitrogen monoxide gas, a nitrous oxide gas, an ozone gas, watervapor, and a mixed gas of oxygen and hydrogen. With the gas containingoxygen, an inert gas such as a nitrogen gas or a rare gas may beintroduced.

In the case where oxygen is added by an ion implantation method, thedose of the oxygen 111 is preferably greater than or equal to 1×10¹³ions/cm² and less than or equal to 5×10¹⁶ ions/cm². With such a dose, ametal oxide film containing oxygen satisfying the stoichiometricproportion or a metal oxide film containing oxygen that is more thanoxygen satisfying the stoichiometric proportion can be formed.

In the case where oxygen is added by plasma treatment, oxygen in oxygenplasma is added to the metal film and the oxide insulating film 107. Inthe case where oxygen is added by plasma treatment, a plasma treatmentapparatus such as a plasma CVD apparatus or a dry etching apparatus canbe used. In the case where a plasma treatment apparatus is used, themetal film 109 is oxidized and part of oxygen is added to the oxideinsulating film 107; thus, a support base or an electrode on which thesubstrate 101 is mounted is preferably biased. As a result, the oxygen111 having energy, typically an oxygen ion, can be drawn on thesubstrate 101 side, and the amount of oxygen added to the oxideinsulating film 112 and the metal oxide film 113 can be furtherincreased.

Further, in the case where oxygen is added by plasma treatment, bymaking oxygen excited by a microwave to generate high density oxygenplasma, the amount of oxygen added to the oxide insulating film 112 towhich oxygen is added can be increased and the dense metal oxide film113 can be formed. Note that in the case where oxygen is made to beexcited by a microwave to generate high density oxygen plasma, theoxygen-adding treatment is preferably performed at temperature at whichoxygen is not released from the oxide insulating film and the oxidesemiconductor film, typically, lower than or equal to 250° C.,preferably lower than or equal to 200° C.

Addition of oxygen into the metal film by plasma treatment leads toimprovement in throughput.

Further, formation of the metal film 109 and oxygen-adding treatment onthe metal film 109 can be performed in the same apparatus. Typically,after the metal film 109 is formed by introducing an inert gas into atreatment chamber in a sputtering apparatus and sputtering a target,oxygen is introduced into the treatment chamber, a support base or anelectrode on which the substrate 101 is mounted is biased, and oxygen,typically an oxygen ion, is drawn to the metal film, so that oxygen canbe added to the metal film to form the metal oxide film and added alsoto the oxide insulating film 112. Further, alternate switching of gasesadded to the chamber enables formation of the metal film and addition ofoxygen into the metal film to be performed alternately, so that themetal oxide film can be thicker.

A state of the oxygen addition into the metal film 109 and the oxideinsulating film 107 is described here with reference to FIGS. 3A and 3B.FIG. 3A corresponds to FIG. 2C, and FIG. 3B corresponds to FIG. 2D.Oxygen is added to the oxide insulating film 107 and the metal film 109,so that the oxygen 111 is added to the metal film 109 and the oxideinsulating film 107 as illustrated in FIGS. 3A and 3B. The oxygen 111 isadded to the oxide semiconductor film 105. At this time, by the oxygenaddition, oxygen vacancies in the side surface of the oxidesemiconductor film 105 are compensated. The oxygen 111 is added to theoxide insulating film 103. The oxygen added to the oxide insulating film103 is diffused into the oxide insulating film 103 as shown in thehorizontal arrows. The diffused oxygen is diffused into the oxidesemiconductor film 105, so that oxygen vacancies in the oxidesemiconductor film 105 are compensated.

After that, heat treatment may be performed. The heat treatment can makethe bond of metal atoms and oxygen in the metal oxide film 113 furtherstrengthened, and oxygen release from the metal oxide film 113 can besuppressed in later heat treatment. The heating temperature at this timeis higher than or equal to 300° C. and lower than or equal to 500° C.,preferably higher than or equal to 400° C. and lower than or equal to450° C.

Oxygen may be added to the oxide insulating film 107 before the metalfilm 109 is formed. As a result of performing oxygen addition pluraltimes, the amount of oxygen in the oxide insulating film 112 to whichoxygen is added can be further increased.

Here, as the oxide insulating film 107, a silicon oxynitride film havinga thickness of 20 nm is formed by a CVD method. Next, as the metal film109, an aluminum film having a thickness of 10 nm is formed by asputtering method. Subsequently, as the oxygen-adding treatmentperformed on the metal film 109, an inductively coupled plasma method isused. The power of the electrode facing the substrate 101 is 0 W, thepower of the electrode on which the substrate 101 is provided is 4500 W,the pressure of the treatment chamber in the plasma generation treatmentapparatus is 15 Pa, and the flow rate of oxygen to be introduced intothe treatment chamber is 250 sccm, so that oxygen plasma is generated.The metal film 109 is exposed to the oxygen plasma to be oxidized, sothat an aluminum oxide film is formed.

As an insulating film in contact with the oxide semiconductor film 105,the oxide insulating film 112 to which oxygen is added is provided. Themetal oxide film 113 is provided over the insulating film. The metaloxide film 113 formed using aluminum oxide, aluminum oxynitride, galliumoxide, gallium oxynitride, yttrium oxide, yttrium oxynitride, hafniumoxide, or hafnium oxynitride has a blocking effect against oxygen,hydrogen, water, and the like; thus, oxygen can be efficiently diffusedfrom the oxide insulating film 112 to which oxygen is added to the oxidesemiconductor film 105. The oxygen 111 is added to the oxidesemiconductor film 105 not by direct addition but by solid-phasediffusion from the oxide insulating film 112 to which oxygen is added;thus, the oxide semiconductor film 105 can be less damaged. Inparticular, the oxygen vacancies in a side surface of the oxidesemiconductor film are compensated by diffusion of the oxygen. As aresult, leakage current between the source electrode and the drainelectrode flowing through the side surfaces, which are shown by a dashedline 129 in FIG. 1A, of the oxide semiconductor film overlapping withthe gate electrode 115 can be reduced.

In a CAAC-OS film, oxygen tends to move along the surface where theCAAC-OS film is formed or the surface of the CAAC-OS film. Thus, oxygenrelease occurs from the side surface of the oxide semiconductor film 105that is subjected to element isolation, and oxygen vacancies tend to beformed in the side surface. By forming an oxide insulating film over theoxide semiconductor film 105 and a metal oxide film over the oxideinsulating film, oxygen release from the side surface of the oxidesemiconductor film 105 can be suppressed. As a result, an increase inconductivity of the side surface of the oxide semiconductor film 105 canbe suppressed.

Next, as illustrated in FIGS. 2G and 2H, the gate electrode 115 and theinsulating film 116 are formed over the metal oxide film 113.

The gate electrode 115 and the insulating film 116 are formed asfollows. A conductive film and an insulating film are stacked, and amask is formed over the insulating film by a photolithography process.Then, part of the insulating film is etched using the mask to form theinsulating film 116. Subsequently, the conductive film is etched usingthe insulating film 116 as a hard mask to form the gate electrode 115.

The conductive film to be the gate electrode 115 is formed by asputtering method, a CVD method, an evaporation method, or the like.

The insulating film to be the insulating film 116 is formed by asputtering method, a CVD method, an evaporation method, or the like.

A tantalum nitride film with a thickness of 30 nm and a tungsten filmwith a thickness of 200 nm are formed here by a sputtering method. Next,a silicon nitride film with a thickness of 50 nm is formed by a CVDmethod. Then, a mask is formed by a photolithography process and thetantalum nitride film, the tungsten film, and the silicon nitride filmare dry-etched with use of the mask to form the gate electrode 115 andthe insulating film 116.

Next, by using the gate electrode 115 as a mask, dopants 117 are addedto the oxide semiconductor film 105 (see FIGS. 2G and 2H), so that thepair of second regions 119 b containing the dopants are formed asillustrated in FIG. 4B. Since the dopants are added with use of the gateelectrode 115 as a mask, the pair of second regions 119 b containing thedopants and the first region 119 a into which the dopants are not addedcan be formed in a self-aligned manner (see FIGS. 4A and 4B). The firstregion 119 a overlapping with the gate electrode 115 serves as a channelregion. Part of the pair of second regions 119 b containing the dopantsserves as a source region and a drain region. The first region 119 a andthe pair of second regions 119 b containing the dopants are referred toas the oxide semiconductor film 119.

As a method for adding the dopants to the oxide semiconductor film 119,an ion doping method or an ion implantation method can be used. As theadded dopants, at least one of boron, nitrogen, phosphorus, and arseniccan be added. Alternatively, as the dopants, at least one of helium,neon, argon, krypton, and xenon can be added. Still alternatively, asthe dopants, at least one of boron, nitrogen, phosphorus, and arsenicand at least one of helium, neon, argon, krypton, and xenon inappropriate combination can be added.

In the embodiment describe here, the addition of the dopants into theoxide semiconductor film 119 is conducted in a state where the oxidesemiconductor film 119 is covered with the insulating film and the like;alternatively, the addition of the dopants may be conducted in a statewhere the oxide semiconductor film 119 is exposed.

Alternatively, the dopants can be added by a method other than injectionmethods such as an ion doping method and an ion implantation method. Forexample, the dopants can be added in the following manner: plasma isgenerated under an atmosphere of gas containing an element to be addedand plasma treatment is performed on a film to which the dopants areadded. A dry etching apparatus, a plasma CVD apparatus, a high-densityplasma CVD apparatus, or the like can be used to generate the plasma.

The addition treatment of the dopants may be performed while heattreatment is performed.

By adding the dopants to the oxide semiconductor film 119, the secondregions 119 b can be amorphous. Since hydrogen tends to be diffused intoan amorphous oxide semiconductor, hydrogen in the first region 119 a isdiffused into the second regions 119 b so that the hydrogenconcentration in the first region 119 a, which is to be a channelregion, can be reduced and the second regions 119 b can be an n-type(have a low resistance). As a result, on-state current of the transistorcan be increased.

Here, phosphorus is added to the oxide semiconductor film 119 by an ionimplantation method.

After that, heat treatment may be performed. The heat treatment isperformed typically at a temperature higher than or equal to 150° C. andlower than or equal to 450° C., preferably higher than or equal to 250°C. and lower than or equal to 325° C. In the heat treatment, thetemperature may be gradually increased from 250° C. to 325° C.

Through the heat treatment, the resistance of the pair of second regions119 b containing dopants can be reduced. In the heat treatment, the pairof second regions 119 b containing the dopants may be in either acrystalline state or an amorphous state.

Next, as illustrated in FIGS. 4C and 4D, the sidewall insulating films121 on the side surfaces of the gate electrode 115 and the gateinsulating film 123 are formed. A method for forming the sidewallinsulating films 121 is described below.

First, an insulating film that is to be the sidewall insulating film 121is formed over the gate insulating film 123 and the gate electrode 115.The insulating film is formed by a sputtering method, a CVD method, orthe like. In addition, although the thickness of the insulating film isnot particularly limited, the thickness is selected as appropriate inconsideration of coverage with respect to the shape of the gateelectrode 115.

Next, the sidewall insulating film 121 is formed by etching theinsulating film. The sidewall insulating film 121 can be formed in aself-aligned manner by performing a highly anisotropic etching step onthe insulating film. Here, dry etching is preferably employed as highlyanisotropic etching, and a gas including fluorine such astrifluoromethane (CHF₃), octafluorocyclobutane (C₄F₈), ortetrafluoromethane (CF₄) can be used as an etching gas. A rare gas suchas helium (He) or argon (Ar) or hydrogen (H₂) may be added to theetching gas. In addition, as the dry etching, a reactive ion etching(RIE) method in which high-frequency voltage is applied to a substrate,is preferably used.

The insulating film 116 is formed using an insulating film having higheretching selectivity than the insulating film to be the sidewallinsulating films 121, so that the insulating film 116 functions as anetching protective film of the gate electrode 115, which is preferable.

Here, a silicon oxynitride film having a thickness of 90 nm is formed bya CVD method. Next, a silicon oxynitride film that is the oxideinsulating film 112 and an aluminum oxide film that is the metal oxidefilm 113 are dry-etched, so that the sidewall insulating films 121 areformed. Note that here the insulating film 116 is formed using, forexample, a silicon nitride film having different compositions from thatof the sidewall insulating films and etching conditions with highselectivity are selected, so that the insulating film 116 can functionas an etching protective film of the gate electrode 115.

Since, in the cross section, the width of each of the sidewallinsulating films 121 may depend on the thickness of the gate electrode115, the thickness of the gate electrode 115 is set to make the width ofeach of the sidewall insulating films 121 be in a desired range.

At the same time that the sidewall insulating films 121 are formed, theoxide insulating film 112 and the metal oxide film 113 are etched byhighly anisotropic etching to expose the oxide semiconductor film 119,so that the gate insulating film 123 can be formed.

Next, as illustrated in FIGS. 4E and 4F, the pair of electrodes 125 isformed.

After a conductive film is formed by a sputtering method, a CVD method,an evaporation method, or the like, a mask is formed over the conductivefilm and the conductive film is etched, thereby the pair of electrodes125 is formed. The mask formed over the conductive film can be formed bya printing method, an inkjet method, or a photolithography method asappropriate. In the case of forming the pair of electrodes 125 with useof a mask, the mask is removed later.

The pair of electrodes 125 is preferably formed to be in contact with atleast side surfaces of the sidewall insulating films 121 and the gateinsulating film 123. In other words, it is preferable that the endportions of the pair of electrodes 125 of the transistor be located overat least the sidewall insulating films 121 and that the pair ofelectrodes 125 entirely covers exposed portions of the pair of secondregions 119 b containing the dopants in the oxide semiconductor film119. Further, the end portions of the pair of electrodes 125 may belocated over the insulating film 116. As a result, regions 119 b 1 incontact with the pair of electrodes 125 serve as a source region and adrain region in the pair of second regions 119 b containing the dopants.In addition, with regions 119 b 2 overlapping with the sidewallinsulating films 121 and the gate insulating film 123 in the pair ofsecond regions 119 b containing the dopants, a source-drain breakdownvoltage can be increased. Further, the distance between a source and adrain can be adjusted depending on the lengths of the sidewallinsulating films 121; thus, end portions of the electrodes 125 on thechannel side, which are in contact with the oxide semiconductor film119, can be formed without a mask. Further, since a mask is not used,variation of a plurality of transistors due to processing can bereduced.

Here, a tungsten film with a thickness of 30 nm is formed by asputtering method. Next, a mask is formed over the tungsten film by aphotolithography process and the tungsten film is dry-etched with use ofthe mask to form the pair of electrodes 125.

After the pair of electrodes 125 is formed, cleaning treatment ispreferably performed to remove an etching residue. A short circuit ofthe pair of electrodes 125 can be suppressed by this cleaning treatment.The cleaning treatment with a solution can be performed using analkaline solution such as a tetramethylammonium hydroxide (TMAH)solution, an acidic solution such as a diluted hydrofluoric acidsolution or an oxalic acid solution, or water.

Next, the insulating film 127 is formed as illustrated in FIGS. 4E and4F.

The insulating film 127 is formed by a sputtering method, a CVD method,a coating method, a printing method, or the like.

Oxygen may be added to the insulating film 127 by an ion implantationmethod, an ion doping method, a plasma method, or the like.

Here, a silicon oxynitride film having a thickness of 460 nm is formedby a CVD method as the insulating film 127.

After that, heat treatment may be performed. The heat treatment isperformed typically at a temperature higher than or equal to 150° C. andlower than the strain point of the substrate, preferably higher than orequal to 250° C. and lower than or equal to 450° C., more preferablyhigher than or equal to 300° C. and lower than or equal to 450° C.

An electric furnace, an RTA apparatus, or the like can be used for theheat treatment. With use of an RTA apparatus, heat treatment at atemperature higher than or equal to the strain point of the substratecan be performed only for a short time. Thus, time during which hydrogenor water is released from the oxide insulating film can be shortened.

The heat treatment may be performed under an atmosphere of nitrogen,oxygen, ultra-dry air (air in which a water content is 20 ppm or less,preferably 1 ppm or less, more preferably 10 ppb or less), or a rare gas(argon, helium, or the like). The atmosphere of nitrogen, oxygen,ultra-dry air, or a rare gas preferably does not contain hydrogen,water, and the like.

Here, the heating is performed under an oxygen atmosphere at 400° C. foran hour with an electric furnace.

Through the above-described steps, the transistor can be manufactured.

According to this embodiment, a metal film is formed over an oxideinsulating film, and oxygen can be added to the metal film to form ametal oxide film and added also to the oxide insulating film. The oxideinsulating film and the metal oxide film into which oxygen is suppliedare used as a gate insulating film of the transistor including the oxidesemiconductor film. Since oxygen is added to the oxide insulating filmin contact with the oxide semiconductor film, the oxygen can be diffused(solid-phase diffusion) into the oxide semiconductor film by heattreatment, so that oxygen vacancies in the oxide semiconductor film canbe reduced. In particular, oxygen vacancies in the side surface of theoxide semiconductor film are compensated by diffusion of the oxygen, sothat generation of a parasitic channel can be suppressed. As a result,leakage current between the source electrode and the drain electrodeflowing through the side surfaces of the oxide semiconductor filmoverlapping with the gate electrode can be reduced.

The oxide semiconductor film is covered with the oxide insulating filmin which part of oxygen is released by heating and the oxide insulatingfilm to which oxygen is added. Therefore, in the heat treatment, oxygencontained in the oxide insulating film can be efficiently diffused intothe oxide semiconductor film, so that oxygen vacancies in the oxidesemiconductor film and the vicinity of the interface between the oxidesemiconductor film and the oxide insulating film can be reduced.

Therefore, the negative shift of the threshold voltage of the transistorcan be reduced and a leakage current between a source and a drain of thetransistor can be reduced; accordingly, electrical characteristics ofthe transistor can be improved. With such structures of the transistorand the periphery thereof (including the base insulating film), thechannel length of the transistor can be less than or equal to 100 nm,e.g., 30 nm. Also in such a case, the off-state current density (a valueobtained by dividing the off-state current by the channel width of thetransistor) can be several yA/μm to several zA/μm.

Since the metal oxide film is formed by adding oxygen to the metal film,productivity can be improved.

(Embodiment 2)

In this embodiment, a transistor in which a metal oxide film formed byoxidizing a metal film is used as a gate insulating film and a methodfor manufacturing the transistor are described with reference to FIGS.1A to 1C, FIGS. 2A to 2H, FIGS. 3A and 3B, FIGS. 4A to 4F, and FIGS. 5Aand 5B. The transistor in this embodiment is different from that inEmbodiment 1 in that only the metal oxide film formed by adding oxygento a metal film is the gate insulating film.

FIGS. 5A and 5B are cross-sectional views of the transistor described inthis embodiment. The top view of the transistor illustrated in FIGS. 5Aand 5B is similar to that in FIG. 1A described in Embodiment 1, and thusis not illustrated here. FIG. 5A is a cross-sectional view in thechannel width direction of the transistor, which corresponds to thedashed-dotted line A-B in FIG. 1A. FIG. 5B is a cross-sectional view inthe channel length direction of the transistor, which corresponds to thedashed-dotted line C-D in FIG. 1A.

The transistor illustrated in FIGS. 5A and 5B includes the oxideinsulating film 103 over the substrate 101, the oxide semiconductor film119 over the oxide insulating film 103, the pair of electrodes 125 thatis in contact with the oxide semiconductor film 119 and functions as asource electrode and a drain electrode, a gate insulating film 153 incontact with at least part of the oxide semiconductor film 119, and thegate electrode 115 provided over the gate insulating film 153 to overlapwith the oxide semiconductor film 119.

Further, the transistor includes the sidewall insulating films 121 incontact with the side surfaces of the gate electrode 115. The oxidesemiconductor film 119 includes the first region 119 a overlapping withthe gate electrode 115 and the pair of second regions 119 b betweenwhich the first region 119 a is interposed and that contains thedopants. The second regions 119 b are low-resistance regions. In theoxide semiconductor film 119, the first region 119 a serves as a channelregion, and regions of the pair of second regions 119 b containing thedopants that are in contact with the pair of electrodes 125 serve as asource region and a drain region. The transistor may include theinsulating film 127 over the oxide insulating film 103, the gateelectrode 115, the sidewall insulating films 121, and the pair ofelectrodes 125. The insulating film 116 is preferably provided betweenthe gate electrode 115 and the insulating film 127.

In the transistor illustrated in FIGS. 5A and 5B, a metal oxide filmformed by adding oxygen to a metal film is used as the gate insulatingfilm 153.

Next, a method for manufacturing the transistor illustrated in FIGS. 5Aand 5B is described. In the transistor illustrated in FIGS. 5A and 5B, ametal film is formed over the oxide semiconductor film 105 after thesteps for the transistor in FIGS. 2A and 2B described in Embodiment 1.The metal film can be formed as appropriate using a material and aformation method that are similar to those of the metal film 109described in Embodiment 1.

Next, as in Embodiment 1, oxygen is added to the metal film to oxidizethe metal film and added also to the oxide semiconductor film.

Since oxygen is added to the metal film and added also to the oxideinsulating film in contact with the oxide semiconductor film, oxygenvacancies in the oxide semiconductor film can be reduced. In particular,oxygen vacancies in a side surface of the oxide semiconductor film arecompensated by diffusion of the oxygen, so that generation of aparasitic channel can be suppressed. As a result, leakage currentbetween the source electrode and the drain electrode flowing through theside surfaces, which are shown by the dashed line 129 in FIG. 1A, of theoxide semiconductor film overlapping with the gate electrode can bereduced.

Next, heat treatment may be performed.

Here, a method for forming the metal film and a method of adding oxygento the metal film can be similar to those in Embodiment 1.

After that, as in Embodiment 1, steps illustrated in FIGS. 2G and 2H andFIGS. 4A to 4F are performed, so that the transistor illustrated inFIGS. 5A and 5B can be manufactured.

Through the above-described steps, a transistor in which the negativeshift of the threshold voltage is reduced and a leakage current betweena source and a drain is reduced and that has excellent electricalcharacteristics can be manufactured with high productivity.

(Embodiment 3)

In this embodiment, a transistor in which a metal oxide film formed byoxidizing a metal film is used as a protective film for the transistorand a method for manufacturing the transistor are described withreference to FIGS. 1A to 1C, FIGS. 4A to 4F, and FIGS. 6A and 6B.

FIGS. 6A and 6B are cross-sectional views of the transistor described inthis embodiment. The top view of the transistor illustrated in FIGS. 6Aand 6B is similar to that in FIG. 1A described in Embodiment 1, and thusis not illustrated here. FIG. 6A is a cross-sectional view in thechannel width direction of the transistor, which corresponds to thedashed-dotted line A-B in FIG. 1A. FIG. 6B is a cross-sectional view inthe channel length direction of the transistor, which corresponds to thedashed-dotted line C-D in FIG. 1A.

The transistor illustrated in FIGS. 6A and 6B includes the oxideinsulating film 103 over the substrate 101, the oxide semiconductor film119 over the oxide insulating film 103, the pair of electrodes 125 thatis in contact with the oxide semiconductor film 119 and functions as asource electrode and a drain electrode, a gate insulating film 163 incontact with at least part of the oxide semiconductor film 119, and thegate electrode 115 provided over the gate insulating film 163 to overlapwith the oxide semiconductor film 119. Further, the transistor includesthe sidewall insulating films 121 in contact with the side surfaces ofthe gate electrode 115. Moreover, the transistor includes a protectivefilm 167 over the oxide insulating film 103, the gate electrode 115, thesidewall insulating films 121, and the pair of electrodes 125.

The oxide semiconductor film 119 includes the first region 119 aoverlapping with the gate electrode 115 and the pair of second regions119 b between which the first region 119 a is interposed and thatcontains the dopants. The second regions 119 b are low-resistanceregions. In the oxide semiconductor film 119, the first region 119 aserves as a channel region, and regions of the pair of second regions119 b containing the dopants that are in contact with the pair ofelectrodes 125 serve as a source region and a drain region. Theinsulating film 116 is preferably provided between the gate electrode115 and an oxide insulating film 167 a.

In the transistor illustrated in FIGS. 6A and 6B, the protective film167 has a staked-layer structure of the oxide insulating film 167 a anda metal oxide film 167 b. The oxide insulating film 167 a is formed onthe gate electrode 115 side. The metal oxide film 167 b is in contactwith the oxide insulating film 167 a, and is a metal oxide film formedby adding oxygen to a metal film.

The gate insulating film 163 has a stacked-layer structure of a firstgate insulating film 163 a in contact with the oxide semiconductor film119 and a second gate insulating film 163 b in contact with the gateelectrode 115. The material and the formation method of the oxideinsulating film 123 a described in Embodiment 1 can be used for thefirst gate insulating film 163 a as appropriate. The material and theformation method of the metal oxide film 123 b described in Embodiment 1can be used for the second gate insulating film 163 b as appropriate.For the second gate insulating film 163 b, a CVD method or a sputteringmethod may be applied as appropriate as an alternative to the formationmethod described in Embodiment 1.

The gate insulating film 163 having a stacked-layer structure isdescribed here; however, the gate insulating film 163 can have asingle-layer structure as in the gate insulating film 153 described inEmbodiment 2 and the material and the formation method of the gateinsulating film 153 can be used as appropriate. A CVD method or asputtering method may be applied as appropriate as an alternative to theformation method described in Embodiment 2.

Next, a method for manufacturing the transistor in FIGS. 6A and 6B isdescribed. In the transistor illustrated in FIGS. 6A and 6B, after thesteps for the transistor in FIGS. 2A to 2H and FIGS. 4A to 4D describedin Embodiment 1, an oxide insulating film 164 is formed, and a metalfilm 165 is formed over the oxide insulating film 164 as illustrated inFIGS. 7A and 7B.

The oxide insulating films 164 may be formed with a single layer or astack including one or more of silicon oxide, silicon oxynitride,silicon nitride oxide, silicon nitride, aluminum oxide, aluminumoxynitride, aluminum nitride oxide, aluminum nitride, and the like.

The oxide insulating films 164 is formed by a sputtering method, a CVDmethod, a coating method, a printing method, or the like.

The metal film 165 can be formed as appropriate using a material and aformation method that are similar to those of the metal film 109described in Embodiment 1.

Next, as in Embodiment 1, the oxygen 111 is added to the metal film 165to oxidize the metal film 165 and added also to the oxide insulatingfilm 164, whereby the oxide insulating film 167 a and the metal oxidefilm 167 b to which oxygen is added are formed as illustrated in FIGS.7C and 7D.

Oxygen may be added to the oxide insulating film 164 before the metalfilm 165 is formed. As a result of performing oxygen addition pluraltimes, the amount of oxygen in the oxide insulating film 164 to whichoxygen is added can be further increased.

Here, as the oxide insulating film 164, a silicon oxynitride film havinga thickness of 50 nm is formed by a CVD method. Next, as the metal film165, an aluminum film having a thickness of 10 nm is formed by asputtering method. Then, oxygen-adding treatment is performed on themetal film 165 using the similar conditions to Embodiment 1, so that analuminum oxide film is formed.

A state of oxygen addition into the metal film 165 and the oxideinsulating film 164 in the case where an oxide insulating film such as asilicon oxide film or a silicon oxynitride film is used as theinsulating film 116 is described here with reference to FIGS. 8A and 8B.FIG. 8A corresponds to FIG. 7A, and FIG. 8B corresponds to FIG. 7B.Oxygen is added to the oxide insulating film 164 and the metal film 165,so that the oxygen 111 is added to the metal film 165 and the oxideinsulating film 164 as illustrated in FIGS. 7A and 7B. The oxygen 111 isdiffused into the sidewall insulating films 121 through the insulatingfilm 116. As a result, the sidewall insulating films 121 containsoxygen. The oxygen 111 is added to the oxide insulating film 103. Theoxygen added to the oxide insulating film 103 is diffused into the oxideinsulating film 103 as shown in the horizontal arrows. The diffusedoxygen is diffused into the oxide semiconductor film 105, so that oxygenvacancies in the oxide semiconductor film 105 are compensated.

After that, heat treatment may be performed. The heating temperature istypically higher than or equal to 150° C. and lower than the strainpoint of the substrate, preferably higher than or equal to 300° C. andlower than or equal to 500° C., more preferably higher than or equal to400° C. and lower than or equal to 450° C. As a result of performingoxygen addition plural times, the amount of oxygen in the oxideinsulating film 164 to which oxygen is added can be further increased.

An electric furnace, an RTA apparatus, or the like can be used for theheat treatment. With use of an RTA apparatus, heat treatment at atemperature higher than or equal to the strain point of the substratecan be performed only for a short time.

The heat treatment may be performed under an atmosphere of nitrogen,oxygen, ultra-dry air (air in which a water content is 20 ppm or less,preferably 1 ppm or less, more preferably 10 ppb or less), or a rare gas(argon, helium, or the like). The atmosphere of nitrogen, oxygen,ultra-dry air, or a rare gas preferably does not contain hydrogen,water, and the like.

Here, the heat treatment is performed under an oxygen atmosphere at 400°C. for an hour with an electric furnace.

The oxide insulating film 167 a to which oxygen is added is providedover the transistor and the metal oxide film 167 b is provided over theoxide insulating film 167 a. The metal oxide film functions as an oxygendiffusion prevention film; thus, oxygen contained in the oxideinsulating film 103 can be efficiently diffused into the oxidesemiconductor film in heat treatment. Further, oxygen in the oxideinsulating film 167 a can be efficiently diffused into the oxidesemiconductor film through the oxide insulating film 103. The metaloxide film also functions as a film for preventing entry of hydrogen,water, and the like; thus, entry of hydrogen, water, and the like fromthe outside to the oxide semiconductor film in the transistor can besuppressed. Thus, leakage current of the transistor can be reduced.

Through the above-described steps, the transistor illustrated in FIGS.6A and 6B can be manufactured.

Through the above-described steps, a transistor in which the negativeshift of the threshold voltage is reduced and a leakage current betweena source and a drain is reduced and that has excellent electricalcharacteristics can be manufactured with high productivity.

(Embodiment 4)

In this embodiment, a transistor in which a metal oxide film formed byoxidizing a metal film is used as a protective film of the transistorand a method for manufacturing the transistor are described withreference to FIGS. 1A to 1C, FIGS. 2A to 2H, FIGS. 3A and 3B, FIGS. 4Ato 4F, and FIGS. 9A and 9B. The transistor in this embodiment isdifferent from that in Embodiment 3 in that only the metal oxide filmformed by adding oxygen to a metal film is the protective film of thetransistor.

FIGS. 9A and 9B are cross-sectional views of the transistor described inthis embodiment. The top view of the transistor illustrated in FIGS. 9Aand 9B is similar to that in FIG. 1A described in Embodiment 1, and thusis not illustrated here. FIG. 9A is a cross-sectional view in thechannel width direction of the transistor, which corresponds to thedashed-dotted line A-B in FIG. 1A. FIG. 9B is a cross-sectional view inthe channel length direction of the transistor, which corresponds to thedashed-dotted line C-D in FIG. 1A.

The transistor illustrated in FIGS. 9A and 9B includes the oxideinsulating film 103 over the substrate 101, the oxide semiconductor film119 over the oxide insulating film 103, the pair of electrodes 125 thatis in contact with the oxide semiconductor film 119 and functions as asource electrode and a drain electrode, the gate insulating film 163 incontact with at least part of the oxide semiconductor film 119, and thegate electrode 115 provided over the gate insulating film 163 to overlapwith the oxide semiconductor film 119. Further, the transistor includesthe sidewall insulating films 121 in contact with the side surfaces ofthe gate electrode 115. Further, the transistor includes a protectivefilm, that is, a metal oxide film 177 over the oxide insulating film103, the gate electrode 115, the sidewall insulating films 121, and thepair of electrodes 125.

The oxide semiconductor film 119 includes the first region 119 aoverlapping with the gate electrode 115 and the pair of second regions119 b between which the first region 119 a is interposed and thatcontains dopants. The second regions 119 b are low-resistance regions.In the oxide semiconductor film 119, the first region 119 a serves as achannel region, and regions of the pair of second regions 119 bcontaining dopants that are in contact with the pair of electrodes 125serve as a source region and a drain region. The insulating film 116 ispreferably provided between the gate electrode 115 and the metal oxidefilm 177.

In the transistor illustrated in FIGS. 9A and 9B, the metal oxide film177 has a single-layer structure and is a metal oxide film formed byadding oxygen to a metal film.

Next, a method for manufacturing the transistor illustrated in FIGS. 9Aand 9B is described. In the transistor illustrated in FIGS. 9A and 9B, ametal film is formed over the insulating film 116 and the sidewallinsulating films 121 after the steps for the transistor in FIGS. 2A to2H and FIGS. 4A to 4D described in Embodiment 1. The metal film can beformed as appropriate using a material and a formation method that aresimilar to those of the metal film 109 described in Embodiment 1.

Next, as in Embodiment 1, oxygen is added to the metal film to form themetal oxide film 177 by oxidizing the metal film and added also to theoxide semiconductor film. After that, heat treatment may be performed.

Here, as in Embodiment 1, a metal film is formed and oxygen-addingtreatment is performed, so that the metal oxide film 177 is formed.

An insulating film may be formed over the metal oxide film 177 with athickness of greater than or equal to 100 nm and less than or equal to500 nm, preferably greater than or equal to 200 nm and less than orequal to 400 nm.

After that, heat treatment may be performed as in Embodiment 3.

Here, the heat treatment is performed under an oxygen atmosphere at 400°C. for an hour with an electric furnace.

The metal oxide film 177 is provided over the transistor. The metaloxide film functions as an oxygen diffusion prevention film; thus,oxygen contained in the oxide insulating film 103 can be efficientlydiffused into the oxide semiconductor film in the heat treatment. Themetal oxide film also functions as a film for preventing entry ofhydrogen, water, and the like; thus, entry of hydrogen, water, and thelike from the outside to the oxide semiconductor film in the transistorcan be suppressed. Thus, leakage current of the transistor can bereduced.

Through the above-described steps, the transistor illustrated in FIGS.9A and 9B can be manufactured.

Through the above-described steps, a transistor in which the negativeshift of the threshold voltage is reduced and a leakage current betweena source and a drain is reduced and that has excellent electricalcharacteristics can be manufactured with high productivity.

(Embodiment 5)

In this embodiment, a method for manufacturing a transistor including apair of electrodes whose shape is different from that of the pair ofelectrodes described in Embodiment 1 is described with reference toFIGS. 2A to 2H, FIGS. 4A to 4F, and FIGS. 10A to 10C. In thisembodiment, description is made using Embodiment 1; however, thisembodiment can also be applied to Embodiments 2 to 4 as appropriate.

After the steps for the transistor in FIGS. 2A to 2H and FIGS. 4A to 4Das in Embodiment 1, a conductive film 181 is formed, and a resist 183 isapplied on the conductive film 181 as illustrated in FIG. 10A.

Since the conductive film 181 is to be the pair of electrodes, thematerial of the pair of electrodes 125 can be applied to the conductivefilm 181 as appropriate. The conductive film 181 is formed by asputtering method, a CVD method, an evaporation method, or the like.

Next, as illustrated in FIG. 10B, the resist 183 is processed to exposethe conductive film 181, whereby a mask 185 is formed.

A first method for forming the mask 185 is a method in which the resist183 is heated and then is etched entirely to expose the conductive film181. Wet etching or dry etching may be employed as appropriate foretching the heated resist.

A second method for forming the mask 185 is a method in which the resist183 is entirely exposed and then is developed. A region where the gateelectrode 115 and the sidewall insulating films 121 are formed is convexhere, and thus the thickness of a resist over the region is small. Theresist 183 is entirely exposed with the amount of light exposure suchthat the resist of the region is removed and a resist over the secondregions 119 b in the oxide semiconductor film remains, whereby the mask185 exposing part of the conductive film 181 can be formed.

Next, as illustrated in FIG. 10C, the conductive film 181 is etchedusing the mask 185, so that a pair of electrodes 187 can be formed.

A divided pair of electrodes can be formed by forming an insulating filmover the conductive film 181, instead of the resist 183, by a CVD methodor a coating method, and then subjecting the insulating film and theconductive film 181 to chemical mechanical polishing treatment. In thecase of the process, by forming the insulating film 116 using aninsulating film that is not likely to be polished by chemical mechanicalpolishing treatment, the conductive film 181 can be divided selectivelyand etching of the gate electrode 115 can be prevented.

According to this embodiment, a mask for forming the pair of electrodes187 can be formed without a photomask; thus, a transistor having aminiaturized structure can be manufactured with high yield regardless ofalignment accuracy of a photomask and accuracy of processing techniquesuch as reduced projection exposure technique.

(Embodiment 6)

In this embodiment, a structure of a transistor including an oxidesemiconductor film whose shape is different from that of the oxidesemiconductor film described in Embodiment 1 and a manufacturing methodthereof are described with reference to FIGS. 2A to 2H, FIGS. 4A to 4F,and FIG. 11. In this embodiment, description is made using Embodiment 1;however, this embodiment can also be applied to Embodiments 2 to 5 asappropriate.

Side surfaces of an oxide semiconductor film 191 of the transistorillustrated in FIG. 11 are substantially aligned with those of the gateinsulating film 123. Further, the side surfaces of the oxidesemiconductor film 191 are in contact with a pair of electrodes 193.Even in the case where the contact area between the oxide semiconductorfilm 191 and the pair of electrodes 193 is smaller than the contact areabetween the oxide semiconductor film and the pair of electrodes in anyof Embodiments 1 to 5, the oxide semiconductor film can be electricallyconnected to the pair of electrodes because the oxide semiconductor film191 has high conductivity.

Next, a method for manufacturing the transistor illustrated in FIG. 11is described.

After the steps for the transistor in FIGS. 2A to 2H and FIGS. 4A to 4Das in Embodiment 1, the oxide semiconductor film 119 is etched with useof the insulating film 116, the sidewall insulating films 121, and thegate insulating film 123 as masks, so that the oxide semiconductor film191 illustrated in FIG. 11 is formed. Wet etching or dry etching can beemployed as appropriate for etching of the oxide semiconductor film 119.

After the steps illustrated in FIGS. 4E and 4F, the transistorillustrated in FIG. 11 can be manufactured.

The transistor described in this embodiment can make the area of theoxide semiconductor film smaller. Thus, a transistor having a smallerarea than the transistor in any of Embodiments 1 to 5 can bemanufactured and thus, higher integration of a semiconductor device canbe achieved.

(Embodiment 7)

In this embodiment, a structure including an insulating film covering atransistor, contact plugs connected to a pair of electrodes of thetransistor through the insulating film, and wirings connected to thecontact plugs is described with reference to FIG. 12. Note that in thisembodiment, description is made using Embodiment 1; however, thisembodiment can also be applied to Embodiments 2 to 5 as appropriate.

An insulating film 201 is provided over the insulating film 127 coveringthe transistor illustrated in FIG. 12. Contact plugs 203 in contact withthe pair of electrodes 125 of the transistor are provided in openingportions of the insulating film 201. Wirings 205 in contact with thecontact plugs 203 are provided over the insulating film 201. Thesurfaces of the insulating film 201 and the contact plugs 203 areplanarized. Thus, other semiconductor elements can be stacked over theinsulating film 201, which enables high integration.

The material of the insulating film 127 can be applied to the insulatingfilm 201 as appropriate. Note that the thickness of the insulating film201 is set such that the pair of electrodes 125 is not exposed whenplanarization treatment is performed in a later step. The material usedfor the pair of electrodes 125 can be applied to the contact plugs 203and the wirings 205 as appropriate.

Next, a method for manufacturing the transistor in FIG. 12 is described.

An insulating film to be the insulating film 201 later is formed overthe insulating film 127 by a CVD method, a coating method, a printingmethod, or the like. Next, part of the insulating film 127 and part ofthe insulating film are etched, so that the opening portions reachingthe pair of electrodes 125 are formed.

When the two opening portions between which the gate electrode 115 isprovided are individually formed, the distance between the openingportions can be shorter than the resolution limit of a light exposureapparatus. For example, one of the opening portions is formed so as toas close to the gate electrode 115 as possible and then the other of theopening portions is formed so as to as close to the gate electrode 115as possible in a similar manner. By making the distance between theopening portions shorter than the resolution limit of a light exposureapparatus by such a method, a transistor that can be furtherminiaturized can be formed particularly in the case where a slimmingprocess is performed at the time of processing the gate electrode 115,because the width of the gate electrode 115 is smaller than theresolution limit of a light exposure apparatus.

Note that a process for forming the opening portions one by one isdescribed here; however, they may be formed at a time.

Next, a conductive film is formed over the insulating film in which theopening portions are formed. The thickness of the conductive film is setsuch that the opening portions can be filled when planarizationtreatment is performed in a later step.

Then, planarization treatment is performed on the upper portion of theinsulating film and the conductive film, so that the insulating film 201and the contact plugs 203 are formed. The planarization treatment isperformed to a depth at which the conductive film is divided.

Subsequently, a conductive film is formed over the insulating film 201and the contact plugs 203. Then, a mask is formed over the conductivefilm by a photolithography process and part of the conductive film isetched with use of the mask to form the wirings 205.

The contact plugs 203 and the wirings 205 may be formed by a damascenemethod as appropriate.

Through the above-described steps, the transistor illustrated in FIG. 12can be manufactured.

(Embodiment 8)

In this embodiment, a method for lowering the resistance of a sourceregion and a drain region in the oxide semiconductor film is describedwith reference to FIGS. 2A to 2H, FIGS. 4A to 4F, and FIGS. 13A to 13C.Note that in this embodiment, description is made using Embodiment 1;however, this embodiment can also be applied to Embodiments 2 to 5 asappropriate.

After the steps for the transistor in FIGS. 2A to 2H and FIGS. 4A to 4Das in Embodiment 1, a metal film 211 is formed over the insulating film116, the oxide semiconductor film 119, the sidewall insulating films121, and the gate insulating film 123 that are illustrated in FIG. 13A.

For the metal film 211, a metal film containing one or more metalelements selected from aluminum, indium, titanium, tin, molybdenum,tungsten, zinc, hafnium, tantalum, lanthanum, barium, magnesium,zirconium, and nickel can be used.

The metal film 211 is formed by a CVD method, a sputtering method, anevaporation method, or the like. The thickness of the metal film 211 maybe greater than or equal to 1 nm and less than or equal to 30 nm, morepreferably greater than or equal to 2 nm and less than or equal to 5 nm.

Next, heat treatment is performed under an oxygen atmosphere, a nitrogenatmosphere, an inert gas atmosphere, or a reduced-pressure atmosphere.The heating temperature at this time may be set to be higher than orequal to 100° C. and lower than or equal to 700° C., preferably higherthan or equal to 200° C. and lower than or equal to 400° C.

By the heat treatment, in a region where the metal film 211 is incontact with the oxide semiconductor film 119, metal elements in themetal film 211 are diffused into the oxide semiconductor film 119 andoxygen contained in the oxide semiconductor film 119 is diffused into ametal film 122, so that oxygen vacancies are formed in the oxidesemiconductor film 119.

As a result, as illustrated in FIG. 13B, an oxide semiconductor film 213is formed. The oxide semiconductor film 213 includes a first region 213a overlapping with the gate electrode 115, a pair of second regions 213b that contains the dopants and between which the first region 213 a isinterposed, and third regions 213 c that contains the dopants and metalelements and between which the pair of second regions 213 b isinterposed. Note that the second regions 213 b have a lower resistancethan the first region 213 a functioning as a channel region. Since thethird regions 213 c that contains the dopants and metal elements and inwhich oxygen vacancies are formed, the third regions 213 c have a lowerresistance than the second regions 213 b.

In the conditions of the above heat treatment, the metal film 211 isoxidized to be a metal oxide film in some cases. In FIG. 13B, a metaloxide film 215 formed by oxidizing the metal film is illustrated.

Next, the metal oxide film 215 is removed, so that the third regions 213c in the oxide semiconductor film 213 are exposed as illustrated in FIG.13C. Depending on the conditions of the heat treatment, the metal film211 is not oxidized in some cases, in which case the metal film 211 isremoved.

After that, steps illustrated in FIGS. 4E and 4F are performed, so thatthe transistor including the third regions having a lower resistancethan the second regions described in Embodiment 1 in the oxidesemiconductor film functioning as the source region and the drain regioncan be manufactured. As a result, a transistor with high on-statecurrent can be manufactured.

(Embodiment 9)

In this embodiment, an example of a method for forming a gate electrodewhose width is reduced to a length shorter than or equal to theresolution limit of a light exposure apparatus is described withreference to FIGS. 2A to 2H and FIGS. 14A to 14D. Note that in thisembodiment, description is made using Embodiment 1; however, thisembodiment can also be applied to Embodiments 2 to 8 as appropriate. Thetransistor described in this embodiment has a different structure fromthe transistor described in Embodiment 1 in a structure of the gateelectrode.

After the steps illustrated in FIGS. 2A to 2H as in Embodiment 1, a gateelectrode 221 whose cross-sectional shape is triangle is formed over themetal oxide film 113 as illustrated in FIG. 14A. Next, an insulatingfilm 225 is formed over the metal oxide film 113 and the gate electrode221.

Here, a method for forming the gate electrode 221 whose cross-sectionalshape is triangle is described. A slimming process is preferablyperformed on a mask used for forming the gate electrode 221 to make themask have a further miniaturized structure. As the slimming process, anashing process using an oxygen radical or the like can be employed, forexample. However, the slimming process other than the ashing process maybe used as long as the mask formed by a photolithography method or thelike can be processed to have a further miniaturized structure. Sincethe channel length of a transistor is determined by the mask formed bythe slimming process, a process with high controllability is preferablyemployed. As a result of the slimming process, the width of the maskformed by a photolithography method or the like can be reduced to alength shorter than or equal to the resolution limit of a light exposureapparatus, preferably less than or equal to half of the resolution limitof a light exposure apparatus, and more preferably less than or equal toone third of the resolution limit of the light exposure apparatus. Forexample, the width of the formed mask can be greater than or equal to 30nm and less than or equal to 2000 nm, preferably greater than or equalto 50 nm and less than or equal to 350 nm. By performing etching on theconductive film while the slimmed mask is made to recede, the gateelectrode 221 whose cross-sectional shape is triangle can be formed.

The material of the insulating film 127 can be applied to the insulatingfilm 225 as appropriate. Note that the thickness of the insulating film225 is set such that the gate electrode 221 is not exposed whenplanarization treatment is performed in a later step.

Next, as illustrated in FIG. 14B, planarization treatment is performedon the insulating film 225 to form an insulating film 227 whose surfaceis flat. Then, a mask 229 is formed over the insulating film by aphotolithography process. Here, since the surface of the insulating film227 is flat, the mask 229 having a miniaturized structure can be formed.

Then, part of the oxide insulating film 112, part of the metal oxidefilm 113, and part of the insulating film 227 are etched with use of themask 229 to form a gate insulating film 233 including an oxideinsulating film 233 a and a metal oxide film 233 b and a sidewallinsulating film 231 as illustrated in FIG. 14C. Note that here, thesidewall insulating film 231 covers not only side surfaces of the gateelectrode 221 but also a top portion thereof.

Subsequently, a pair of electrodes 235 and an insulating film 237 areformed as illustrated in FIG. 14D. Note that the pair of electrodes 235and the insulating film 237 can be formed in a similar manner to thepair of electrodes 125 and the insulating film 127, respectively,described in Embodiment 1.

Through the above-described steps, a transistor including a gateelectrode whose width is reduced to a length shorter than or equal tothe resolution limit of a light exposure apparatus can be manufactured.

(Embodiment 10)

In this embodiment, an example of a semiconductor device that includesany of the transistors described in this embodiment, is capable ofholding stored data even when not powered, and does not have alimitation on the number of write cycles is described with reference todrawings. Note that here, a memory device is described as an example ofa semiconductor device.

FIGS. 15A to 15C illustrate an example of a structure of a semiconductordevice. FIG. 15A is a cross-sectional view of the semiconductor device,FIG. 15B is a top view of the semiconductor device, and FIG. 15C is acircuit diagram of the semiconductor device. Here, FIG. 15A correspondsto a cross section along line C1-C2 and line D1-D2 in FIG. 15B. Notethat in FIG. 15B, some components of the semiconductor device (e.g., asubstrate 300, a gate insulating film 308, an insulating film 328, aninsulating film 329, an insulating film 330, a gate insulating film 346,an interlayer insulating film 335, an insulating film 349, an insulatingfilm 350, an insulating film 352, and a wiring 356) are omitted forclarity.

The semiconductor device illustrated in FIGS. 15A and 15B includes atransistor 360 including a first semiconductor material in a lowerportion, and a transistor 362 including a second semiconductor materialin an upper portion. The transistor 362 is an example having a similarstructure to that of any of the transistors described in Embodiments 1to 9 as appropriate.

Here, the first semiconductor material and the second semiconductormaterial are preferably materials having different band gaps. Forexample, the first semiconductor material may be a semiconductormaterial other than an oxide semiconductor (e.g., silicon) and thesecond semiconductor material may be an oxide semiconductor. Atransistor including a material other than an oxide semiconductor canoperate at high speed easily. On the other hand, charge can be held in atransistor including an oxide semiconductor for a long time owing to itscharacteristics.

Although all the transistors are n-channel transistors here, it isneedless to say that p-channel transistors can be used. Any of thetransistors described in Embodiments 1 to 9, which includes an oxidesemiconductor film, is used so that data can be held. A specificstructure of the semiconductor device, such as a material of thesemiconductor device or a structure of the semiconductor device, is notlimited to the structure described here.

The transistor 360 in FIG. 15A includes a channel region 316 provided inthe substrate 300 containing a semiconductor material (e.g., silicon),impurity regions 320 provided so that the channel region 316 issandwiched therebetween, intermetallic compound regions 324 in contactwith the impurity regions 320, the gate insulating film 308 providedover the channel region 316, and a gate electrode 310 provided over thegate insulating film 308. Note that a transistor whose source electrodeand drain electrode are not illustrated in a drawing may be referred toas a transistor for the sake of convenience. Further, in such a case, indescription of a connection of a transistor, a source region and asource electrode are collectively referred to as a “source electrode,”and a drain region and a drain electrode are collectively referred to asa “drain electrode.” That is, in this specification, the term “sourceelectrode” may include a source region.

Further, an element isolation insulating film 306 is provided on thesubstrate 300 to surround the transistor 360, and the insulating film328 and the insulating film 330 are provided to cover the transistor360. In the transistor 360, the sidewall insulating films may be formedon side surfaces of the gate electrode 310, and the impurity regions 320may include regions having different impurity element concentrations.

The transistor 360 formed using a single crystal semiconductor substratecan operate at high speed. Thus, when the transistor is used as areading transistor, data can be read at a high speed. Two insulatingfilms are formed so as to cover the transistor 360. As treatment priorto formation of the transistor 362 and a capacitor 364, CMP treatment isperformed on the two insulating films, whereby the insulating film 328and the insulating film 330 that are planarized are formed and, at thesame time, a top surface of the gate electrode 310 is exposed.

The insulating film 328, the insulating film 329, and the insulatingfilm 330 can be formed using an inorganic insulating film: typicalexamples of the inorganic insulating film are a silicon oxide film, asilicon oxynitride film, an aluminum oxide film, an aluminum oxynitridefilm, a silicon nitride film, an aluminum nitride film, a siliconnitride oxide film, and an aluminum nitride oxide film, and the like.The insulating film 328 and the insulating film 330 can be formed by aplasma CVD method, a sputtering method, or the like.

Alternatively, an organic material such as polyimide, an acrylic resin,or a benzocyclobutene resin can be used. Other than such organicmaterials, it is also possible to use a low-dielectric constant material(a low-k material) or the like. In the case of using an organicmaterial, the insulating film 328 and the insulating film 330 may beformed by a wet method such as a spin coating method or a printingmethod.

In this embodiment, the insulating film 328 is formed using a siliconnitride film, the insulating film 329 is formed using a metal oxidefilm, typically an aluminum oxide film, functioning as a diffusionprevention film of water, hydrogen, oxygen, or the like, and theinsulating film 330 is formed using an oxide insulating film describedin Embodiment 1, typically a silicon oxide film, from which part ofoxygen is released by heating. With such a structure, hydrogen containedin the silicon nitride film is diffused into the substrate 300 includinga semiconductor material, specifically into the channel region 316, in alater heating step so that the vacancies of the region can behydrogenated, and oxygen contained in the silicon oxide film is diffusedinto an oxide semiconductor film 344 so that oxygen vacancies in theoxide semiconductor film 344 can be reduced.

Planarization treatment is preferably performed on the surface of theinsulating film 330. In this embodiment, the oxide semiconductor film344 is formed over the insulating film 330 that is sufficientlyplanarized by polishing treatment such as CMP treatment (the averagesurface roughness of the surface of the insulating film 330 ispreferably less than or equal to 0.15 nm).

The transistor 362 illustrated in FIG. 15A includes the oxidesemiconductor film 344, a pair of electrodes 342 a and 342 b in contactwith the oxide semiconductor film 344 and functions as a sourceelectrode and a drain electrode, the gate insulating film 346 in contactwith at least part of the oxide semiconductor film 344, and a gateelectrode 348 that is located over the gate insulating film 346 andoverlaps with oxide semiconductor film 344. The transistor 362 furtherincludes the insulating film 349 over the gate electrode 348 andsidewall insulating films 336 a and 336 b in contact with side surfacesof the gate electrode 348.

Any of the transistors described in Embodiments 1 to 9, which includesan oxide semiconductor in a channel region, can be used as thetransistor 362 as appropriate. The transistor 362 has a short channellength. The length is set greater than or equal to 5 nm and smaller than60 nm, preferably greater than or equal to 10 nm and smaller than orequal to 40 nm. Since the transistor 362 includes an oxide semiconductorfilm in a channel region, the transistor 362 does not have a shortchannel effect or has less short channel effect, and shows favorableelectrical characteristics as a switching element.

Since the off-state current of the transistor 362 is small, stored datacan be held for a long time owing to the transistor. In other words,power consumption can be sufficiently reduced because a semiconductordevice in which refresh operation is unnecessary or the frequency ofrefresh operation is extremely low can be provided.

The interlayer insulating film 335 and the insulating film 350 eachhaving a single-layer structure or a stacked-layer structure areprovided over the transistor 362. In this embodiment, an aluminum oxidefilm is used as the insulating film 350. The density of the aluminumoxide film is made to be high (the film density is higher than or equalto 3.2 g/cm³, preferably higher than or equal to 3.6 g/cm³), wherebystable electrical characteristics can be given to the transistor 362.

In addition, a conductive film 353 is provided in a region overlappingwith the electrode 342 a of the transistor 362 with the interlayerinsulating film 335 and the insulating film 350 provided therebetween,and the electrode 342 a, the interlayer insulating film 335, theinsulating film 350, and the conductive film 353 form the capacitor 364.That is, the electrode 342 a of the transistor 362 functions as oneelectrode of the capacitor 364, and the conductive film 353 functions asthe other electrode of the capacitor 364. Note that the capacitor 364may be omitted if a capacitor is not needed. Alternatively, thecapacitor 364 may be separately provided above the transistor 362.

The insulating film 352 is provided over the transistor 362 and thecapacitor 364. Furthermore, the wiring 356 for connecting the transistor362 to another transistor is provided over the insulating film 352.Although not illustrated in FIG. 15A, the wiring 356 is electricallyconnected to the electrode 342 b through an electrode formed in anopening portion provided in the insulating film 350, the insulating film352, the gate insulating film 346, and the like. Here, the electrode ispreferably provided so as to partly overlap with at least the oxidesemiconductor film 344 of the transistor 362.

In FIGS. 15A and 15B, the transistor 360 and the transistor 362 areprovided so as to at least partly overlap with each other. The sourceregion or the drain region of the transistor 360 is preferably providedso as to overlap with part of the oxide semiconductor film 344.Furthermore, the transistor 362 and the capacitor 364 are provided so asto overlap with at least part of the transistor 360. For example, theconductive film 353 of the capacitor 364 is provided so as to overlapwith at least part of the gate electrode 310 of the transistor 360. Withsuch a planar layout, the area occupied by the semiconductor device canbe reduced; thus, higher integration can be achieved.

Note that the electrical connection between the electrode 342 b and thewiring 356 may be established by direct contact of the electrode 342 band the wiring 356 with each other or through an electrode provided inan insulating film lying between the electrode 342 b and the wiring 356.Alternatively, the electrical connection may be established through aplurality of electrodes.

Next, an example of a circuit configuration corresponding to FIGS. 15Aand 15B is illustrated in FIG. 15C.

In FIG. 15C, a first wiring (1st line) is electrically connected to asource electrode of the transistor 360. A second wiring (2nd line) iselectrically connected to a drain electrode of the transistor 360. Athird wiring (3rd line) is electrically connected to one of a sourceelectrode and a drain electrode of the transistor 362, and a fourthwiring (4th line) is electrically connected to a gate electrode of thetransistor 362. A gate electrode of the transistor 360 and the one ofthe source electrode and the drain electrode of the transistor 362 areelectrically connected to the one electrode of the capacitor 364. Afifth line (5th line) is electrically connected to the other electrodeof the capacitor 364.

The semiconductor device in FIG. 15C utilizes a characteristic in whichthe potential of the gate electrode of the transistor 360 can be held,and thus can write, hold, and read data as follows.

Writing and holding of data is described. First, the potential of thefourth line is set to a potential at which the transistor 362 is turnedon, so that the transistor 362 is turned on. Accordingly, the potentialof the third line is supplied to the gate electrode of the transistor360 and the capacitor 364. That is, predetermined charge is given to thegate electrode of the transistor 360 (writing). Here, charge for supplyof a potential level or charge for supply of a different potential level(hereinafter referred to as Low level charge and High level charge) isgiven. After that, the potential of the fourth line is set to apotential at which the transistor 362 is turned off, so that thetransistor 362 is turned off. Thus, the charge given to the gateelectrode of the transistor 360 is held (holding).

Since the off-state current of the transistor 362 is extremely low, thecharge of the gate electrode of the transistor 360 is held for a longtime.

Next, reading of data is described. By supplying an appropriatepotential (reading potential) to the fifth line while a predeterminedpotential (constant potential) is supplied to the first line, thepotential of the second line varies depending on the amount of chargeheld in the gate electrode of the transistor 360. This is because ingeneral, when the transistor 360 is an n-channel transistor, an apparentthreshold voltage V_(th) _(—) _(H) in the case where a high-level chargeis given to the gate electrode of the transistor 360 is lower than anapparent threshold voltage V_(th) _(—) _(L) in the case where alow-level charge is given to the gate electrode of the transistor 360.Here, an apparent threshold voltage refers to the potential of the fifthline, which is needed to turn on the transistor 360. Thus, the potentialof the fifth line is set to a potential V₀ that is between V_(th) _(—)_(H) and V_(th) _(—) _(L), whereby charge given to the gate electrode ofthe transistor 360 can be determined. For example, in the case where ahigh-level charge is given in writing, when the potential of the fifthwiring is set to V₀ (>V_(th) _(—) _(H)), the transistor 360 is turnedon. In the case where a low level charge is given in writing, even whenthe potential of the fifth wiring is set to V₀ (<V_(th) _(—) _(L)), thetransistor 360 remains in an off state. Therefore, the held data can beread by the potential of the second line.

Note that in the case where memory cells are arrayed to be used, onlydata of desired memory cells needs to be read. In the case where suchreading is not performed, a potential at which the transistor 360 isturned off, that is, a potential smaller than V_(th) _(—) _(H) may begiven to the fifth wiring regardless of the state of the gate electrodeof the transistor 360. Alternatively, a potential which allows thetransistor 360 to be turned on regardless of a state of the gateelectrode, that is, a potential higher than V_(th) _(—) _(L) may beapplied to the fifth line.

When a transistor including an oxide semiconductor film in having achannel region and having extremely small off-state current is appliedto the semiconductor device in this embodiment, the semiconductor devicecan hold stored data for an extremely long period. In other words, powerconsumption can be adequately reduced because refresh operation becomesunnecessary or the frequency of refresh operation can be extremely low.Moreover, stored data can be held for a long period even when power isnot supplied (note that a potential is preferably fixed).

Furthermore, in the semiconductor device described in this embodiment,high voltage is not needed for writing data and there is no problem ofdeterioration of elements. For example, unlike a conventionalnon-volatile memory, it is not necessary to inject and extract electronsinto and from a floating gate, and thus a problem such as deteriorationof a gate insulating film does not occur at all. In other words, thesemiconductor device according to one embodiment of the presentinvention does not have a limit on the number of times of writing whichis a problem in a conventional non-volatile memory, and reliabilitythereof is drastically improved. Furthermore, data is written dependingon the on state and the off state of the transistor, whereby high-speedoperation can be easily realized.

As described above, a miniaturized and highly-integrated semiconductordevice having stable and excellent electrical characteristics and amethod for manufacturing the semiconductor device can be provided.

The methods and structures described in this embodiment can be combinedas appropriate with any of the methods and structures described in theother embodiments.

(Embodiment 11)

In this embodiment, a semiconductor device which includes the transistordescribed in Embodiment 1 to 9, which can hold stored data even when notpowered, which does not have a limitation on the number of write cycles,and which has a structure different from the structure described inEmbodiment 10 is described with reference to FIGS. 16A and 16B and FIGS.17A and 17B. Note that here, a memory device is described as an exampleof a semiconductor device.

FIG. 16A illustrates an example of a circuit configuration of asemiconductor device, and FIG. 16B is a conceptual diagram illustratingan example of a semiconductor device. First, the semiconductor deviceillustrated in FIG. 16A is described, and then, the semiconductor deviceillustrated in FIG. 16B is described.

In the semiconductor device illustrated in FIG. 16A, a bit line BL iselectrically connected to a source electrode or a drain electrode of thetransistor 362, a word line WL is electrically connected to a gateelectrode of the transistor 362, and the source electrode or the drainelectrode of the transistor 362 is electrically connected to a firstterminal of a capacitor 454.

Next, writing and holding of data in the semiconductor device (a memorycell 450) illustrated in FIG. 16A will be described.

First, the potential of the word line WL is set to a potential at whichthe transistor 362 is turned on, so that the transistor 362 is turnedon. Accordingly, the potential of the bit line BL is supplied to thefirst terminal of the capacitor 454 (writing). After that, the potentialof the word line WL is set to a potential at which the transistor 362 isturned off, so that the transistor 362 is turned off. Thus, thepotential of the first terminal of the capacitor 454 is held (holding).

Off current is extremely small in the transistor 362 including an oxidesemiconductor film. For that reason, the potential of the first terminalof the capacitor 454 (or a charge accumulated in the capacitor 454) canbe held for an extremely long period by turning off the transistor 362.

Next, reading of data is described. When the transistor 362 is turnedon, the bit line BL which is in a floating state and the capacitor 454are electrically connected to each other, and the charge isredistributed between the bit line BL and the capacitor 454. As aresult, the potential of the bit line BL is changed. The amount ofchange in potential of the bit line BL varies depending on the potentialof the first terminal of the capacitor 454 (or the charge accumulated inthe capacitor 454).

For example, the potential of the bit line BL obtained after chargeredistribution is (CB×B0+C×V)/(CB+C), where V is the potential of thefirst terminal of the capacitor 454, C is the capacitance of thecapacitor 454, CB is the capacitance of the bit line BL (hereinafteralso referred to as bit line capacitance), and VBO is the potential ofthe bit line BL obtained before the charge redistribution. Therefore, itcan be found that assuming that the memory cell 450 is in either of twostates in which the potentials of the first terminal of the capacitor454 are V1 and V0 (V1>V0), the potential of the bit line BL in the caseof holding the potential V1 (=(CB×VB0+C×V1)/(CB+C)) is higher than thepotential of the bit line BL in the case of holding the potential V0(=(CB×VB0+C×V0)/(CB+C)).

Then, by comparing the potential of the bit line BL with a predeterminedpotential, data can be read.

As described above, the semiconductor device illustrated in FIG. 16A canhold charge that is accumulated in the capacitor 454 for a long timebecause the off-state current of the transistor 362 is extremely small.In other words, power consumption can be adequately reduced becauserefresh operation becomes unnecessary or the frequency of refreshoperation can be extremely low. Moreover, stored data can be held for along time even when power is not supplied.

Next, the semiconductor device illustrated in FIG. 16B is described.

The semiconductor device illustrated in FIG. 16B includes memory cellarrays 451 a and 451 b including a plurality of memory cells 450illustrated in FIG. 16A as memory circuits in an upper portion, and aperipheral circuit 453 in a lower portion which is necessary foroperating memory cell arrays 451 (the memory cell arrays 451 a and 451b). Note that the peripheral circuit 453 is electrically connected tothe memory cell array 451.

In the structure illustrated in FIG. 16B, the peripheral circuit 453 canbe provided directly under the memory cell arrays 451 (the memory cellarrays 451 a and 451 b). Thus, the size of the semiconductor device canbe decreased.

It is preferable that a semiconductor material of the transistorprovided in the peripheral circuit 453 is different from that of thetransistor 362. For example, silicon, germanium, silicon germanium,silicon carbide, gallium arsenide, or the like can be used, and a singlecrystal semiconductor is preferably used. Alternatively, an organicsemiconductor material or the like may be used. A transistor includingsuch a semiconductor material can operate at sufficiently high speed.Therefore, a variety of circuits (e.g., a logic circuit or a drivercircuit) which needs to operate at high speed can be favorably achievedby using the transistor.

Note that FIG. 16B illustrates, as an example, the semiconductor devicein which the two memory cell arrays 451 (the memory cell arrays 451 aand 451 b) are stacked; however, the number of memory cell arrays to bestacked is not limited thereto. Three or more memory cell arrays may bestacked.

Next, a specific structure of the memory cell 450 illustrated in FIG.16A is described with reference to FIGS. 17A and 17B.

FIGS. 17A and 17B illustrate an example of the structure of the memorycell 450. FIG. 17A is a cross-sectional view of the memory cell 450.FIG. 17B is a top view of the memory cell 450. Here, FIG. 17Acorresponds to a cross-section taken along F1-F2 and G1-G2 in FIG. 17B.Note that in FIG. 15B, illustration of some components of the transistor(e.g., the interlayer insulating film 335, the gate insulating film 346,the insulating film 349, an insulating film 456, an insulating film 458,and a wiring 460) is omitted for clarity.

The transistor 362 illustrated in FIGS. 17A and 17B can have a structuresimilar to that of the transistor described in Embodiment 1 to 9.

The insulating film 456 having a single-layer structure or astacked-layer structure is provided over the transistor 362 over theinsulating film 330. In addition, a conductive film 462 is provided in aregion overlapping with the electrode 342 a of the transistor 362 withthe insulating film 456 interposed therebetween, and the electrode 342a, the interlayer insulating film 335, the insulating film 456, and theconductive film 462 form the capacitor 454. That is, the electrode 342 aof the transistor 362 functions as one electrode of the capacitor 454,and the conductive film 462 functions as the other electrode of thecapacitor 454.

The insulating film 458 is provided over the transistor 362 and thecapacitor 454. Furthermore, the wiring 460 for connecting the memorycell 450 to an adjacent memory cell 450 is provided over the insulatingfilm 458. Although not illustrated, the wiring 460 is electricallyconnected to the electrode 342 b of the transistor 362 through anopening provided in the insulating film 456, the insulating film 458,and the like. The wiring 460 may be electrically connected to theelectrode 342 b through another conductive film provided in the opening.Note that the wiring 460 corresponds to the bit line BL in the circuitdiagram of FIG. 16A.

In FIGS. 17A and 17B, the electrode 342 b of the transistor 362 can alsofunction as a source electrode of a transistor included in an adjacentmemory cell. With such a planar layout, the area occupied by thesemiconductor device can be reduced; thus, higher integration can beachieved.

When the planar layout in FIG. 17B is employed, the area occupied by thesemiconductor device can be reduced; thus, the degree of integration canbe increased.

As described above, the plurality of memory cells formed in multiplelayers in the upper portion is each formed with a transistor includingan oxide semiconductor film. Since the transistor including an oxidesemiconductor film has low off-state current, stored data can be heldfor a long time by using the transistor. In other words, the frequencyof refresh operation can be extremely lowered, which leads to asufficient reduction in power consumption.

A semiconductor device having a novel feature can be obtained by beingprovided with both a peripheral circuit including the transistorincluding a material other than an oxide semiconductor (in other words,a transistor capable of operating at sufficiently high speed) and amemory circuit including the transistor including an oxide semiconductor(in a broader sense, a transistor whose off-state current issufficiently small). In addition, the use of a structure where theperipheral circuit and the memory circuit are stacked leads to anincrease in the degree of integration of the semiconductor device.

As described above, a miniaturized and highly-integrated semiconductordevice having stable and excellent electrical characteristics and amethod for manufacturing the semiconductor device can be provided.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

(Embodiment 12)

As examples of the semiconductor device described in any of the aboveembodiments, a central processing unit, a microprocessor, amicrocomputer, a memory device, an image sensor, an electro-opticaldevice, a light-emitting display device, and the like can be given. Thesemiconductor device can be applied to a variety of electronic devices.Examples of the electronic devices are as follows: display devices,lighting devices, personal computers, word processors, image reproducingdevices, portable compact disc (CD) players, radio receivers, taperecorders, headphone stereos, stereos, clocks, cordless phone handsets,transceivers, portable wireless devices, cellular phones, smart phones,electronic book, car phones, portable game machines, calculators,portable information terminals, e-book readers, electronic translators,audio input devices, cameras such as video cameras or digital stillcameras, electric shavers, high-frequency heating appliances, electricrice cookers, electric washing machines, electric vacuum cleaners, waterheaters, electric fans, hair dryers, air conditioners, humidifiers,dehumidifiers, air-conditioning systems, dishwashing machines, dishdrying machines, clothes dryers, futon dryers, electric refrigerators,electric freezers, electric refrigerator-freezers, freezers forpreserving DNA, flashlights, electric power tools, smoke detectors,medical equipment, guide lights, traffic lights, belt conveyors,elevators, escalators, industrial robots, power storage systems,electric vehicles, hybrid electric vehicles, plug-in hybrid electricvehicles, tracked vehicles, motorized bicycles, motorcycles, electricwheelchairs, golf carts, boats, ships, submarines, helicopters,aircrafts, rockets, artificial satellites, space probes, planetaryprobes, and spacecrafts. In this embodiment, examples of application ofthe semiconductor device described in any of the above embodiments toportable devices such as cellular phones, smartphones, or e-book readersare described with reference to FIGS. 18A and 18B, FIG. 19, FIG. 20, andFIG. 21.

In portable electronic devices such as mobile phones, smartphones, ande-book readers, an SRAM or a DRAM is used to store image datatemporarily. This is because response speed of a flash memory is low andthus a flash memory is not suitable for image processing. On the otherhand, an SRAM or a DRAM has the following characteristics when used fortemporary storage of image data.

In a normal SRAM, as illustrated in FIG. 18A, one memory cell includessix transistors, which are a transistor 801, a transistor 802, atransistor 803, a transistor 804, a transistor 805, and a transistor806, and they are driven by an X decoder 807 and a Y decoder 808. Thetransistors 803 and 805 and the transistors 804 and 806 each serve as aninverter, and high-speed driving can be performed therewith. However, anSRAM has a disadvantage of large cell area because one memory cellincludes six transistors. Provided that the minimum feature size of adesign rule is F, the area of a memory cell in an SRAM is generally 100F² to 150 F². Therefore, a price per bit of an SRAM is the mostexpensive among a variety of memory devices.

On the other hand, as illustrated in FIG. 18B, a memory cell in a DRAMincludes a transistor 811 and a storage capacitor 812, and is driven byan X decoder 813 and a Y decoder 814. One cell includes one transistorand one capacitor and thus the area of a memory cell is small. The areaof a memory cell of a DRAM is generally less than or equal to 10 F².Note that in the case of a DRAM, a refresh operation is always necessaryand power is consumed even when a rewriting operation is not performed.

However, the area of the memory cell of the semiconductor devicedescribed the above embodiments is about 10 F² and frequent refreshingis not needed. Therefore, the area of the memory cell is reduced, andthe power consumption can be reduced.

Next, a block diagram of a portable device is illustrated in FIG. 19. Aportable device illustrated in FIG. 19 includes an RF circuit 901, ananalog baseband circuit 902, a digital baseband circuit 903, a battery904, a power supply circuit 905, an application processor 906, a flashmemory 910, a display controller 911, a memory circuit 912, a display913, a touch sensor 919, an audio circuit 917, a keyboard 918, and thelike. The display 913 includes a display portion 914, a source driver915, and a gate driver 916. The application processor 906 includes acentral processing unit (CPU) 907, a DSP 908, and an interface (IF) 909.In general, the memory circuit 912 includes an SRAM or a DRAM; byemploying the semiconductor device described in any of the aboveembodiments for the memory circuit 912, writing and reading of data canbe performed at high speed, data can be held for a long time, and powerconsumption can be sufficiently reduced. Further, the power consumptionof the CPU 907 can be sufficiently reduced by employing thesemiconductor device described in any of the above embodiments for amain memory device for storing data or an instruction or a buffer memorydevice capable of high-speed writing and reading of data, such as aregister or a cache, which is included in the CPU.

Next, FIG. 20 shows an example in which the semiconductor devicedescribed in any of the above embodiments is used for a memory circuit950 in a display. The memory circuit 950 illustrated in FIG. 20 includesa memory 952, a memory 953, a switch 954, a switch 955, and a memorycontroller 951. Furthermore, the memory circuit is connected to adisplay controller 956 which reads and controls image data input througha signal line (input image data) and data stored in the memories 952 and953 (stored image data), and is also connected to a display 957 whichdisplays an image based on a signal input from the display controller956.

First, image data (input image data A) is formed by an applicationprocessor (not shown). The input image data A is stored in the memory952 though the switch 954. The image data (stored image data A) held inthe memory 952 is transmitted and displayed to the display 957 throughthe switch 955 and the display controller 956.

In the case where the input image data A is not changed, the storedimage data A is read from the memory 952 through the switch 955 by thedisplay controller 956 at a frequency of 30 Hz to 60 Hz in general.

Next, for example, when data displayed on the screen is rewritten by auser (that is, in the case where the input image data A is changed), newimage data (input image data B) is formed by the application processor.The input image data B is held in the memory 953 through the switch 954.The stored image data A is read periodically from the memory 952 throughthe switch 955 even during that time. After the completion of storingthe new image data (the stored image data B) in the memory 953, from thenext frame for the display 957, the stored image data B starts to beread, transmitted to the display 957 through the switch 955 and thedisplay controller 956, and displayed on the display 957. This readingoperation is continued until another new image data is held in thememory 952.

By alternately writing and reading image data to and from the memory 952and the memory 953 as described above, images are displayed on thedisplay 957. Note that the memory 952 and the memory 953 are not limitedto separate memories, and a single memory may be divided and used. Byemploying the semiconductor device described in any of the aboveembodiments for the memory 952 and the memory 953, data can be writtenand read at high speed and held for a long time, and power consumptioncan be sufficiently reduced.

Next, FIG. 21 is a block diagram of an e-book reader. FIG. 21 includes abattery 1001, a power supply circuit 1002, a microprocessor 1003, aflash memory 1004, an audio circuit 1005, a keyboard 1006, a memorycircuit 1007, a touch panel 1008, a display 1009, and a displaycontroller 1010.

Here, the semiconductor device described in any of the above embodimentscan be used for the memory circuit 1007 in FIG. 21. The memory circuit1007 has a function of temporarily holding the contents of a book. Forexample, users may use a highlight function. When users read an e-bookreader, they sometimes want to mark a specified place. This markingrefers to a highlight function, and users can make difference from otherplaces by, for example, changing the color of a letter displayed,underlining a word, making a letter bold, or changing the font type of aletter. That is, there is a function of storing and holding informationof a place specified by users. In order to save information for a longtime, the information may be copied into the flash memory 1004. Even insuch a case, by employing the semiconductor device described in any ofthe above embodiments, writing and reading of data can be performed athigh speed, data can be held for a long time, and power consumption canbe sufficiently reduced.

As described above, the semiconductor device in any of the aboveembodiments is mounted on each of the portable devices described in thisembodiment. Therefore, a portable device in which writing and reading ofdata are performed at high speed, data is held for a long time, andpower consumption is sufficiently reduced, can be obtained.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

This application is based on Japanese Patent Application serial No.2012-013816 filed with Japan Patent Office on Jan. 26, 2012, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A method for manufacturing a semiconductor devicecomprising the steps of: forming an oxide semiconductor film over anoxide insulating film; forming a metal film covering the oxidesemiconductor film; forming a metal oxide film by adding oxygen to themetal film; forming a gate electrode over the metal oxide film; andforming low-resistance regions in the oxide semiconductor film by addingdopants to the oxide semiconductor film with use of the gate electrodeas a mask after the gate electrode is formed, wherein the dopants areone or more of helium, neon, argon, krypton, and xenon, and wherein thedopants are contained in the low-resistance regions at a concentrationof greater than or equal to 5×10¹⁸ atoms/cm³ and smaller than or equalto 1×10²² atoms/cm³.
 2. The method for manufacturing a semiconductordevice, according to claim 1, further comprising the steps of: formingsidewall insulating films in contact with side surfaces of the gateelectrode; etching the metal oxide film not overlapping with the gateelectrode and the sidewall insulating films; and forming a pair ofelectrodes in contact with parts of the oxide semiconductor film and thesidewall insulating films which are exposed by the etching.
 3. Themethod for manufacturing a semiconductor device, according to claim 1,wherein oxygen is added to the metal film and added also to the oxidesemiconductor film.
 4. The method for manufacturing a semiconductordevice, according to claim 1, further comprising the steps of: formingan oxide insulating film over the oxide semiconductor film; and formingthe metal film over the oxide insulating film, wherein oxygen is addedto the metal film and added also to the oxide insulating film over theoxide semiconductor film.
 5. The method for manufacturing asemiconductor device, according to claim 1, wherein the metal film is analuminum film, a hafnium film, or an yttrium film, and wherein the metaloxide film is an oxide film or an oxynitride film corresponding to themetal film.
 6. The method for manufacturing a semiconductor device,according to claim 1, further comprising the steps of: forming sidewallinsulating films in contact with side surfaces of the gate electrode;etching the metal oxide film not overlapping with the gate electrode andthe sidewall insulating films; forming low-resistance regions in theoxide semiconductor film by adding dopants to the oxide semiconductorfilm with use of the gate electrode as a mask after the gate electrodeis formed; and forming the sidewall insulating films over the oxidesemiconductor film including the low-resistance regions, wherein thedopants are one or more of boron, nitrogen, phosphorus, and arsenic. 7.The method for manufacturing a semiconductor device, according to claim1, wherein the oxide semiconductor film comprises one or more elementsselected from the group of In, Ga, Sn, and Zn.
 8. A method formanufacturing a semiconductor device comprising the steps of: forming anoxide semiconductor film over a first oxide insulating film; forming asecond oxide insulating film over the oxide semiconductor film; forminga gate electrode over the second oxide insulating film; forming a metalfilm covering the gate electrode; forming a metal oxide film by addingoxygen to the metal film, and forming low-resistance regions in theoxide semiconductor film by adding dopants to the oxide semiconductorfilm with use of the gate electrode as a mask after the gate electrodeis formed, wherein the dopants are one or more of boron, nitrogen,phosphorus, and arsenic, and wherein the dopants are contained in thelow-resistance regions at a concentration of greater than or equal to5×10¹⁸ atoms/cm³ and smaller than or equal to 1×10²² atoms/cm³.
 9. Themethod for manufacturing a semiconductor device, according to claim 8,further comprising the steps of: forming a third oxide insulating filmover the gate electrode after a pair of electrodes is formed; andforming the metal film covering the third oxide insulating film, whereinoxygen is added to the metal film and added also to the third oxideinsulating film.
 10. The method for manufacturing a semiconductordevice, according to claim 8, wherein the metal film is an aluminumfilm, a hafnium film, or an yttrium film, and wherein the metal oxidefilm is an oxide film or an oxynitride film corresponding to the metalfilm.
 11. The method for manufacturing a semiconductor device, accordingto claim 8, further comprising the steps of: forming low-resistanceregions in the oxide semiconductor film by adding dopants to the oxidesemiconductor film with use of the gate electrode as a mask after thegate electrode is formed; and wherein the dopants are one or more ofhelium, neon, argon, krypton, and xenon.
 12. The method formanufacturing a semiconductor device, according to claim 8, wherein theoxide semiconductor film comprises one or more elements selected fromthe group of In, Ga, Sn, and Zn.
 13. A method for manufacturing asemiconductor device comprising the steps of: forming an oxidesemiconductor film over an oxide insulating film; forming a metal filmcovering the oxide semiconductor film; forming a metal oxide film byadding oxygen to the metal film; forming a gate electrode over the metaloxide film; and forming low-resistance regions in the oxidesemiconductor film by adding dopants to the oxide semiconductor filmwith use of the gate electrode as a mask after the gate electrode isformed, wherein the dopants are one or more of boron, nitrogen,phosphorus, and arsenic, and wherein the dopants are contained in thelow-resistance regions at a concentration of greater than or equal to5×10¹⁸ atoms/cm³ and smaller than or equal to 1×10²² atoms/cm³.
 14. Themethod for manufacturing a semiconductor device, according to claim 13,further comprising the steps of: forming sidewall insulating films incontact with side surfaces of the gate electrode; etching the metaloxide film not overlapping with the gate electrode and the sidewallinsulating films; and forming a pair of electrodes in contact with partsof the oxide semiconductor film and the sidewall insulating films whichare exposed by the etching.
 15. The method for manufacturing asemiconductor device, according to claim 13, wherein oxygen is added tothe metal film and added also to the oxide semiconductor film.
 16. Themethod for manufacturing a semiconductor device, according to claim 13,further comprising the steps of: forming an oxide insulating film overthe oxide semiconductor film; and forming the metal film over the oxideinsulating film, wherein oxygen is added to the metal film and addedalso to the oxide insulating film over the oxide semiconductor film. 17.The method for manufacturing a semiconductor device, according to claim13, wherein the metal film is an aluminum film, a hafnium film, or anyttrium film, and wherein the metal oxide film is an oxide film or anoxynitride film corresponding to the metal film.
 18. The method formanufacturing a semiconductor device, according to claim 13, wherein theoxide semiconductor film comprises one or more elements selected fromthe group of In, Ga, Sn, and Zn.
 19. A method for manufacturing asemiconductor device comprising the steps of: forming an oxidesemiconductor film over a first oxide insulating film; forming a secondoxide insulating film over the oxide semiconductor film; forming a gateelectrode over the second oxide insulating film; forming a metal filmcovering the gate electrode; forming a metal oxide film by adding oxygento the metal film, and forming low-resistance regions in the oxidesemiconductor film by adding dopants to the oxide semiconductor filmwith use of the gate electrode as a mask after the gate electrode isformed, wherein the dopants are one or more of helium, neon, argon,krypton, and xenon, and wherein the dopants are contained in thelow-resistance regions at a concentration of greater than or equal to5×10¹⁸ atoms/cm³ and smaller than or equal to 1×10²² atoms/cm³.
 20. Themethod for manufacturing a semiconductor device, according to claim 19,further comprising the steps of: forming a third oxide insulating filmover the gate electrode after a pair of electrodes is formed; andforming the metal film covering the third oxide insulating film, whereinoxygen is added to the metal film and added also to the third oxideinsulating film.
 21. The method for manufacturing a semiconductordevice, according to claim 19, wherein the metal film is an aluminumfilm, a hafnium film, or an yttrium film, and wherein the metal oxidefilm is an oxide film or an oxynitride film corresponding to the metalfilm.
 22. The method for manufacturing a semiconductor device, accordingto claim 19, wherein the oxide semiconductor film comprises one or moreelements selected from the group of In, Ga, Sn, and Zn.